Whatever happened to the 90-minute film' You only have to look at the top prizes at Venice this year to realise this classic running time is no longer the norm.

The Golden Lion went to Ang Lee's Lust, Caution (156 minutes). The Special Jury prize was shared by The Secret Of The Grain (153 minutes) and I'm Not There (135 minutes) - with the latter also earning Cate Blanchett a Coppa Volpi for best actress. Brad Pitt picked up the best actor award for his performance in The Assassination Of Jesse James By The Coward Robert Ford (159 minutes). Only the Silver Lion for best director went to a film that fitted the traditional paradigm: Brian De Palma's Redacted clocked in at exactly 90 minutes.

Bloated blockbusters

It's not just the arthouse sector that's adding reels. In 2006, the top 10 grossing films in the US averaged 124 minutes; in 1986, the average running time was 108 minutes. And some of the biggest summer blockbusters have conspired to keep that figure high. Pirates Of The Caribbean: At World's End came in at a cramp-inducing 168 minutes. Transformers ran for an entirely unnecessary 144 minutes, while Spider-Man 3 notched up 140 minutes. Harry Potter And The Order Of The Phoenix felt relatively restrained at 138 minutes - 19 minutes shorter than its predecessor, Harry Potter And The Goblet Of Fire.

Even comedies - traditionally one of the genres, along with animation, that stick most closely to the 90-minute rule - are filling out. This year, Hot Fuzz and Knocked Up both clocked in at more than two hours, with Hairspray, Superbad and Run, Fat Boy, Run not far behind.

If things continue like this, exhibitors will soon be providing exercise tips for seat-bound clients, like those handed out on long-haul flights.

Deep-vein thrombosis isn't the only danger. The San Francisco Chronicle ran a great op-ed piece a few years back about the challenges of bladder control in three-hour-plus movies such as The Lord Of The Rings: The Return Of The King. One solution that has been canvassed is the return of the intermission. Interestingly, some cinemas with older single-reel projectors have been forced to reintroduce a mid-movie break for longer films, as the capacity of standard single-reel projection towers is 12,000 feet, or around 133 minutes.

I'm not against length in principle. Though I groan at festivals when I see that a film is so long it's going to make me miss dinner (again), I don't mind grabbing a sandwich on the run (again) if the running time is justified by the outcome. The Departed didn't feel too long at 151 minutes; nor did The Best Of Youth, at 383 minutes.

Short and sweet

What is important, however, is that the final length of the film is weighted to the particular needs of the story. And this can be an issue for traditional-length films just as much as it can for long ones. Recently I've seen a number of films that were stretched at around 90 minutes. Argentinian director Anahi Berneri's fine character study Encarnacion would have been perfect at three-quarters of its 96-minute running time, and if Bela Tarr had restricted himself to an average shot length of 10 seconds, he could have turned his excruciatingly slow 135-minute The Man From London into a peppy 20-minute short.

The best B-movies - like Richard Fleischer's 1952 train-thriller The Narrow Margin - crammed a full cinematic meal into 70 minutes. True, these low-budget titles were warm-ups for the main act, but at a time when people are living increasingly busy lives, is the return of the 70 or even 60-minute feature so unthinkable'