You can't compare any Billboard chart records pre-1991 to post-1991 or pre-1999 to post-1999 because of the way marketing and chart rules have changed.
There's six distinct Billboard chart eras:
Pre-Hot 100 - multiple singles charts for airplay, sales and jukeboxes, often with different Number 1s at once. Pre-rock transitioning to rock (peak Elvis is pre-Hot 100). Long peaks, especially for non-rock songs. Multiple versions of the same song often chart simultaneously (the good original version and the lame Pat Boone cover).
The pre-Hot 100 "Top 100" was dated on Wednesday from 11/2/55 through 6/19/57.
There was a three day "week" with the chart date moving to Saturday on 6/22/57. It stayed Saturday until 7/19/58, the last Top 100.
There was no chart Sat 7/26/58 or Mon 7/28/58.
1958-late 70s - Hot 100 comprehensive chart debuts Monday 8/4/58. It was a Monday date through 12/25/61. It's not entirely clear whether there was a Saturday 12/30/61 chart. If you click on that date on the Billboard website it gives you the 1/6/62 chart. However, some sources list unique data for a 12/30/61 chart. In any case, this was the point of transition from a Monday chart date to the Saturday chart date, where it has remained since.
Data was self-reported by stores and stations, often manipulated (payola). Single release required for Hot 100 eligibility. Some famous album cuts (Stairway To Heaven) ineligible.
Glory days of Top 40 radio. Rapid chart turnover. Short songs, with a norm of 3 minutes, though this grows steadily with time (landmarks: Like A Rolling Stone, Hey Jude. Elton John's singles regularly exceeded 5 minutes.) Albums become important mid to late 60s (Sgt. Pepper). Record sales grow throughout the period. Artists released an album or two (Beatles, Elton John) per year, but usually only one to three singles per album. One off non-album singles are common.
Long chart peaks (I Want To Hold Your Hand 7 weeks at Number 1, Hey Jude 9) fade away around 1970, and vanish by 1976. In 1974-75 one week at Number 1 was the norm and four weeks would get you Number 1 for the year. Chart runs over 20 weeks are rare.
The All Beatle Top Five was a unique anomaly driven by America's delayed case of Beatlemania; rights to the early material were split among multiple labels. Once the early material had all been released in America, this didn't happen again. For my money, given the differences in rules and patterns over the years, the All Beatle Top Five remains the greatest chart achievement.
Another major chart outlier in this era is The Twist due to its two Number 1 chart runs 18 months apart. Until very recently The Twist was called "the biggest hit of all time" (again my whole point is that comparing across eras is impossible).
Late 70s - November 1991- Same chart rules, but different patterns due to different record release strategies. Top 40 Radio fragments and loses its cultural dominance to MTV. Long chart peaks briefly return 1977-82 (You Light Up My Life and Physical 10 weeks, see also Night Fever, Endless Love, I Love Rock & Roll) but vanish by the mid-80s (When Doves Cry at 5 weeks in `84 was about as long as it got - unfortunately for Bruce Springsteen who was stuck at 2 and never did get a Number 1).
Songs start to get longer with the average closer to four minutes; the Casey Kasem countdown expanded from three hours to four.
Record sales plummet fast in the fall of 1978 (notorious flops: Sgt. Pepper Soundtrack, the Kiss solo albums). Albums become more important than singles and cassettes start to take over from vinyl (with CDs emerging late in the era). Sales grow back as the formats change.
Chart runs for singles get a little longer (Tainted Love sets a record at 43 weeks) but rarely get beyond six months. What happens instead:
The Long Album Cycle begins, with acts releasing four (Rumours), five (Purple Rain, Sports, Heartbeat City), or even seven (Born In The USA, Thriller, Rhythm Nation) singles from an album over cycles lasting up to two years. Non-album singles become rare. Still some unity to pop culture (Thriller), with Nirvana being the last mass culture moment.
From 1976 to 1991 the chart was "frozen" over the holiday week. This does not mean "Let It Go" was Number One. The end of the year issue was a double issue focused on year end charts and Billboard skipped a publication week. Officially these unpublished magazine weeks are chart dates, but the chart is identical to the prior week (all positions were "frozen"). In most cases the pre-Christmas #1 song held over, but there were exceptions:
1/1/77: Rod Stewart got credit for an 8th week at #1, a LOT for that era, but fell out of #1 on the 1/8 chart. Did he REALLY hold on for an 8th week or would Marilyn McCoo and Billy Davis Jr have knocked him off a week earlier if there had been a fresh chart?
12/30/78: #1 transition the next week, Le Freak to Too Much Heaven
12/29/79: #1 transition the next week, Pina Colada Song to Please Don't Go.
November 1991-December 5, 1998 - The Soundscan Era, the CD era. A transitional era for charts. Direct collection of data begins in November 1991 and chart patterns change immediately - but single release is still required for Hot 100 eligibility. But this is when the vinyl single dies, and CD and cassette singles never sell as well as the glory years of 45s. Thus radio airplay dominates the charts.
The beginning of extreme long chart runs (How Will I Live, You Were Meant For Me at about 15 months each) and long Number 1 peaks (I Will Always Love You and Macarena at 14 weeks, One Sweet Day at 16).
The album and airplay charts were more accurate indicators of real popularity than the Hot 100 as many key hits (Iris, Don't Speak) were not issued as singles in order to to boost CD sales (the "one good song on the CD" era). This contributed to long #1 peaks as the competition was weaker for the songs that WERE chart eligible (Candle In The Wind 1997 and Unbreak My Heart).
High chart debuts begin in 1995. This culminates in the first Number
1 debuts (MJ again with the quickly forgotten You Are Not Alone).
Debuts were often manipulated by delaying the limited quantity but
chart-required single release until airplay peaked (Sunny Came Home).
Beginning with the weeks of 12/26/92 and 1/2/93 Billboard ended their "holiday freeze" and resumed publishing unique charts over both Christmastime weekends.
December 5, 1998-circa 2010 - Early modern era. Album cuts become chart eligible, and in 2000 Aaliyah earns the first Number 1 single without a single. The iPod and download era (legal and not). With MTV abandoning music videos, and everyone with headphones serving as their own DJ, the mass culture era ends. Still primarily a singles era (begins to change with Taylor Swift's 2008 album Fearless becoming a proto-chart bomb). Number 1 debuts nearly vanish except for American Idol stars.
Several longevity records set, with Black Eyed Peas holding Number 1 for 26 straight weeks.
The 2000s marked the rapid growth of the "Featuring" credit, as collaboration becomes a norm with the singer singing the hook or multiple rappers taking a verse. This skews chart stats a lot, in particular numbers of hits stats.
2010-present - The modern era. Streaming emerges as the primary medium. Number 1 debuts return and become almost the norm. Chart manipulation moves from the payola/false reporting problem of the Classic Hot 100 Era to the fanbase model (bulk purchases, endless loop streaming, etc - Nicki Minaj and KPop fans are particularly rabid at this, though Swifties are not 100% innocent. Many artists play into it). Chart rules frequently change to try to stay one step ahead - see the ban on "bundling" (free single with purchase of tickets or merch).
The "remix" also became a chart factor. This has been around for 20+ years (J Lo's "I'm Real") but in the late 10s and the 20s it's a big chart tactic.
It's not uncommon for a song to reach Number 1 without having ANY impact on the larger pop culture outside the act's core fanbase (the "stans"). Huge Week 2 drops are common (BTS member Jimin's solo single just dropped from 1-45 for a new record. Nicki Minaj dropped 1-34 and Swift dropped 1-38 though that was mostly due to the Holiday Chart Bomb).
The rare "stable and organic" hits stay at Number 1 for months (Old Town Road a record 19 weeks, As It Was 15, Despacito 16, Uptown Funk 14).
Beginning of the serious Chart Bomb Era where many or even all songs on a superstar album (primarily Swift and Drake, but others like Bad Bunny) debut at once. Compare 26 out of 30 tracks on the Hot 100 for Red TV vs. zero of 30 for the no-single White Album. First the Beatles 14 of Hot 100 record fell. Then Drake tied the All Beatle Top Five. Finally Taylor occupied the entire Top Ten.
Most chart bombs are short as the non-singles ("single" being more a mood than an actual format, signaling radio promo efforts or video releases) drop fast the 2nd week (though the biggest tracks from Swift's latest album have shown some longevity in the low to mid rungs).
A minor variation is the Necro Chart Bomb when an artist dies (Prince, Petty, Bowie) and their greatest hits re-chart for a week.
Starting in the late 2010s there's also a four week or so Holiday Chart Bomb when the same core group of Christmas songs returns each year, breaking lots of chart records related to slow climbs, fast drops, and multiple runs (All I Want For Christmas Is You taking 25 years to hit Number 1, hitting Number 1 in four separate chart runs, and dropping from Number 1 to completely off the chart three times).
Each year the Holiday Chart Bomb gets bigger and longer. This has the side effect of interrupting otherwise long consecutive chart streaks, as holiday songs push down non-seasonal songs and trigger the Recurrent Rule (dropping below 50 after 20 weeks gets you dropped from the chart) or the Super Recurrent rule (dropping below 25 after a year). It also leads to high re-entries like "Blinding Lights" returning at number 3 the week after Christmas. I'm not at ALL a fan of the Holiday Chart Bomb (for decades Christmas songs were only listed on a special holiday chart - a policy I believe should return).
Chart turnover is low and slow except for weeks with chart bombs. Songs stay on the chart either one week (Glee Cast, chart bombs), exactly 20 or 52 weeks, or forever: 90 weeks, including over a year in the top ten, for the new "biggest hit of all time" Blinding Lights, and 91 weeks for Heat Waves which took 59 weeks to reach Number 1. Songs peak either in Week 1 or in Week 46 or so, with country tracks lingering for months in the low chart rungs and slowly building.
Some acts accumulate insane numbers of "Featuring" credits (Drake, Minaj, Lil Wayne). In contrast, nearly all of Swift's hits are as a solo artist or in a handful of cases Swift as the lead artist with a guest. However the re-recordings have racked up roughly 40 duplicate hits ("You Belong With Me" and "You Belong With Me Taylor's Version" are considered separate chart entries).
With numbers of streams, and quick hooky viral videos, becoming more
important, the average length of hits has dropped back toward the three
minute mark. One stream of a song is one stream, whether it's the 1:52
original version of Old Town Road or the 10:13 long version of All Too
Well (making Swift's achievement of Number 1, and the longest running
time Number 1 ever, all the more impressive).
