In over a century of cinema, we have seen a wide variety of artistic and technical innovations which have changed the shape (and size) of the silver screen, from the invention of Technicolour and recorded sound to the advent of widescreen and the steadicam. But in terms of the narrative and thematic conventions of the films themselves, several aspects have persisted long after technology has brought them into question. And one of the most persistent of these conventions is the ninety-minute running time.

Ninety minutes is a figure which crops up over and over again in the history of cinema. As silent cinema became a mass medium through the 1910s and 1920s, American distributors selected 90 minutes at the cut-off point for any film being shown in cinemas. If a film were any longer, they reasoned, people would either lose interest and leave, or be put off and not bother to pay in the first place. They stuck so rigidly to this idea that when Metropolis arrived States-side (having already been butchered by the German studios), it was run through projectors at one-and-a-half times the intended speed in certain cities. By the 1950s, trash maestro Roger Corman and his exploitation partners-in-crime were manufacturing 90-minute films for the drive-in and grindhouse circuits, films that were shown in cinemas of endless double bills, designed to do the rounds and then disappear forever.

Nowadays the average film on screen in a multiplex will last the best part of two hours, not including the mind-numbing 20 minutes of adverts and the patronising warnings about piracy. Many films still fall below this running time, and a great many, both arthouse and blockbuster, will beat it. But why does the figure of ninety minutes still hang like a rusty sword of Damacles over the world of filmaking? And what have we lost (or gained) by the now typical disregard for thisconvention?

I count four possible explanations. The first lies in the three-act structure of filmaking, which holds that all films play out (or at least should play out) in three acts: setup, conflict, and resolution. Take Back to the Future as an example - Marty McFly is a teenager living in 1985 (setup); he is transported back to 1955 and cannot return (conflict); he puts things right in the past and finds a way home (resolution). The French writer Yves Lavandier reasoned that in most dramatic works, half the content is taken up with conflict and both setup and resolution take a remaining quarter. We unconsciously expect a certain amount of time to be taken up with each act, and 90 minutes seems a reasonable average to aim for, giving enough scope for storytelling while retaining some form of restrictive discipline.

Regardless of whether Lavandier's maths were right, the three act structure has been called into dispute by a succession of writers and directors. In the last twenty years we have seen films which play with narrative and character identity to such an extent that a simple linear progression of setup-conflict-resolution simply doesn't cut the mustard. Take David Lynch's Mulholland Drive, one of the very best films of the last decade. There is an internal element of the three-act structure - Naomi Watts is an actress in Hollywood (setup); she has a fateful encounter with Rita who has lost her memory (conflict); they sort-of discover who she is (or do they?) (resolution). But this is not an adequate means of summing up an ingeniously complicated plot in which identities change rapidly and time jumps around. Later efforts like Eternal Sunshine of the Spotless Mind and The Prestige raise similar problems.

The second explanation lies in an argument about discipline. Corman famously quipped that no film should be longer than 90 minutes without special Papal dispensation - something which his errant pupil James Cameron swiftly ignored. We can all cite examples of films which are too long - Titanic, Out of Africa and of course Heaven's Gate - but it is extremely rare, if not unheard of, to criticise a film for being too short. If a film is long, it is often seen as indulgent, pretentious or desperately worthy - Mark Kermode once referred to The Deer Hunter (182 mins) as "a cinematic dick-measuring contest". Short films have an in-built modesty to them, a modesty which plays into the film's hands if its story or ideas are remarkably well-told.

This sounds convincing, but one should not simply assume that "short is good, long is bad", to paraphrase the sheep in Animal Farm. While many films are too long, some of the best ever made are well in excess of 90 minutes and most of them justify that length. Imagine trying to cut down Stanley Kubrick's Barry Lyndon to half its length; all the painterly quality which is essential to its success and its unique status would be lost. Both The Shawshank Redemption and The Green Mile thrive on being slow-burners, giving the necessary time for the drama and spiritual insight to emerge naturally, rather than being forced upon an audience. On the other hand, there are a great many short films that are terrible - look at the Troma stable, or its modern equivalent, Asylum Films. Or if you want something a bit more mainstream, I give you Disaster Movie and its associates.

A third explanation as to the value of 90 minutes comes down, rather cynically, to money. Making short films is quick and cheap - it takes less time to write the script, the production costs are lower, and you don't have to spend months locked in an editing suite trying to piece together different scenes from miles of exposed film (or gigabytes of digital footage). In the days of grindhouse and drive-ins, keeping the films short was the only way in which the individual outlets could make money; the double bills had to run to 3 hours, and had to be changed over rapidly in time for the next lot of paying punters to get their few bucks' worth. Working off a low budget focusses screenwriters and filmakers, getting them to say the most they can in the most concise possible way.

That all seems fine, and there are many examples of great low-budget filmaking still going on in Europe, America and elsewhere, from the latest Ryan Reynolds thriller Buried to horror efforts like [REC] and Paranormal Activity. But most of the films which gain wide release are multi-million dollar blockbusters, with budgets of typically more than $100m and a running time of well over 90 minutes. If the 90 minute rule was so reliable, why has it become the trend to make movies by throwing huge amounts of money at them and to hell with the length? The role of producers has changed as well - instead of standing over directors with a big stick asking them to cut it down, the likes of Joel Silver and Jerry Bruckheimer are more concerned nowadays with demographic targeting, demanding that directors deliver a product that will sell regardless of its eventual length.

The fourth and final explanation is probably the most unfair to us punters - limited attention span. Forget all the articles about ADHD or people talking on phones in the cinema - people, or at least certain kinds of people, are very impatient and can very quickly get bored. Although this may seem like a modern phenomenon, it's one thing that hasn't changed from the silent era. We will still fall asleep in films or walk out of them if they go on too long to sustain our attention, and if a film is deliberately billed as a certain length, we probably won't go. Ninety minutes in this context is a happy medium between being bored out of your skull and not getting your money's worth.

But again, here's a problem. If most people won't pay to sit through films which are more than 90 minutes long, how does one explain the huge box office takings for Avatar (162 mins) or the Star Wars prequels (136 mins, 142 mins and 140 mins respectively)? The answer is not a matter of length, and certainly not one of depth. It is a matter of hype; these are event movies, films which exist to be seen at a certain time to separate the in-crowd from the out-crowd. They are films to be watched as an experience, as an "I was there" moment - e.g. "I was there when everyone realised that The Phantom Menace was one of the worst films ever made!" As for keeping their attention, most mega-length, mega-budget movies are packed with action spectacle and technically outstanding set-pieces, designed to keep 14-year-old boys happy and oblivious to their very thin (or non-existent) plot.

In short, there is no one reason why 90 minutes is (or at least was) a touchstone in filmaking. Each explanation has its flaws, and they don't form a coherent thesis when meshed together. But looking around at the erratic quality of filmaking we see today, from Hollywood and elsewhere, maybe certain elements of this rule could be reintroduced and lead to artistic and commercial reinvention further down the line.