In the numerous box office post-mortems that came in the wake of Doctor Sleep, the Warner Bros. follow-up to 1980's The Shining, entertainment industry analysts cited its runtime as one reason the Stephen King adaptation failed to find its footing with viewers in 2019. Projected to earn $30 million on its opening weekend, the two-hour-and-31-minute film apparently kept moviegoers with short attention spans at bay, grossing just $14 million.

But was that really the case? Another King adaptation, It: Chapter Two, came in at nearly three hours and was a major success, making nearly $500 million worldwide after being released in September 2019. Can a movie’s critical and commercial success really be determined by its running time?

For an answer, it helps to understand why filmmakers have, according to data scientist Dr. Randal Olson, seemed to settle in a sweet spot of between 90 and 120 minutes for many major motion pictures. Back in the earliest days of film exhibition, theaters were sent reels in canisters that had to be swapped out by projectionists, with each reel lasting between 11 and 20 minutes. This change was often done manually, which was hard on the projectionists. Theater owners also preferred shorter films, so they could schedule more screenings per day—and thereby make more money. As a result, movies usually weren't longer than two hours.

That changed between the 1930s and 1960s, when theaters and studios were feeling threatened by the advent of television. With attendance dropping, studios felt compelled to make movies more of an event experience, expanding the aspect ratio to a sprawling 1:85:1 or 2:35:1 widescreen format—early films were a boxy 4:3—and offering longer films to help consumers justify a night out at the movies. Epics like 1959’s Ben-Hur (three hours, 44 minutes) and 1962’s Lawrence of Arabia (three hours, 48 minutes) were hits despite their length. During this period, movies gained an average of 30 minutes in their running times, and often had intermissions about two-thirds of the way through.

From 1970 to 1985, however, things changed. With the threat of television fading, movies shrunk by an average of 10 minutes. One theory for the shrinkage is that movies became shorter to fit the standard storage capacity of VHS cassettes, which were increasing in popularity at the time. Ever since, movies have largely remained stable at the sub-120 minute run times, with the average length for a movie in 2018 coming in at a breezy 96.5 minutes.

Even individual shots in movies have shrunk: The length of the average shot before cutting away has fallen from 12.5 seconds in 1930 to 2.5 seconds as of 2010.

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The late film critic Roger Ebert observed this trend, writing in 1992 that a filmgoer’s subconscious had started to expect films to come in no longer than two hours. But he also argued that the idea that films used to be truncated wasn’t always so. Early screenings, after all, featured cartoons, short features, news reels, and other entertainment that could keep people in their seats for hours. Now, recommending a film over that barrier usually leads to complaining. (Try it: Tell a friend a movie you like is nearly three hours long. Watch their eyelids get heavy.)

Is a runtime of under two hours really something we’ve been conditioned to prefer? Some moviegoers think so. A 2015 survey of 1647 British movie fans reported that 55 percent found a movie under two hours to be preferred. But that may depend on the genre. Comedies average 90 minutes in length; dramas tend to run longer. The average running time of a Best Picture winner from 2000 to 2016 was 131 minutes. Which makes it seem as though audiences are willing to accept films running longer if they perceive them as more serious.

Exhibitors still have a say in movie length, too. Longer films tend to mean fewer screenings per day, which means reduced profits. It also means long films get sub-optimal screening times during the day. Ideally, a film should be screened around 8 or 9 p.m. to afford people a chance to see it at a convenient hour. If a film is under two hours, it can also screen at 5:30 and 10:30. But if it’s creeping toward three hours, that means other showtimes begin much earlier or much later than moviegoers prefer. Not many people will opt for a three-hour epic that begins at 11 p.m.

But exhibitors are outside forces motivated by financial interests. At home, there’s no such concern, which would make an entity like Netflix perfect for evaluating audience preferences. The service is famous for analyzing viewership data, which informs many of their acquisition and original programming choices. So what’s the average length of a Netflix-produced film? As of 2017, roughly 97 minutes long—about the average length of all movies in 2018.

While there are always exceptions—Martin Scorsese’s gangster epic The Irishman is three hours and 30 minutes, Quentin Tarantino’s lauded Once Upon a Time in Hollywood is two hours and 41 minutes, and Avengers: Endgame, the highest grossing film of all time, is nearly three hours—such seat-squirming lengths are usually reserved for filmmakers who have earned the confidence of the audience. Seeing a new Scorsese film run long might be a cause for celebration. After viewing, you may think it impossible to be as satisfied if an hour had been hacked from it. But if a first-time director releases a nearly four-hour film, you may be more inclined to skip it.

In the end, it might be best to follow Ebert’s rule of thumb: “Bad movies are always too long," he wrote, "but good movies are either too short, or just right.”