History and reasoning behind average length of a feature film

Thats a very interesting question. Though I couldn't find a precise answer, here are some interesting takes on t he subject that I learned while researching.

From this article on wikipedia:

A feature film is a film that runs for 40 minutes or longer, according to the Academy of Motion Picture Arts and Sciences, American Film Institute, and British Film Institute, though the Screen Actors Guild states that it is 80 minutes or longer.

The majority of feature films are between 90 and 210 minutes long. The Story of the Kelly Gang was the first feature film based on length, and was released in 1906. The first feature-length adaptation was Les Misérables which was released in 1909.

Feature films for children are usually between 60 and 120 minutes.

Cinemablend has an interesting article on this very topic:

There's a school of thought that says 90 minutes is the perfect length for a movie-- the length of 3 TV episodes, just enough time to get in, tell your story, and get out without wasting any more of the audience's time. There are countless examples that prove the rule, economically told stories that feel perfect and tight without a second wasted.

Finally, there is an answer on wikianswers which has a different take:

Most movie scripts are 120 pages and a page translates to roughly a minute of film, hence most films being about just under 2 hours. It's quite genre dependent as comedies and animations are rarely more than 90 minutes, but summer blockbusters and thrillers are usually about 2 hours.

History of theatre:

• Appears the Greeks liked full-length dramas, though the Romans did not; which is to say that the idea of "full-length" is very old, and it's likely impossible to know the reasoning behind story lengths 1000s of years ago.

First feature length films:

• Historic Context: The first feature length film was The Story of the Kelly Gang, which was 60-80 min; the real length is unknown, since only 17-mins of the film still exist today. At the time the first feature length film was introduced, the average film length was 15-mins.

• Reel length: The first feature length film was approximate reel length was 4,000 feet, and at this length would have been a reel-to-reel film; meaning that the duration of the film was not based on the length of the film itself having to be on just one reel. One reason for its length may have been that it was a narrative film, based on a real story; though that's just speculation, and not based on any facts.

• Profits: The Story of the Kelly Gang cost £1,000, was extremely successful, and was said to have returned at least £25,000 to its producers. It's possible that if the first few feature length films had failed financially, there would have been less interest in producing them. It was not until 1911 that countries other than Australia began to make feature films. By this time Australia had made 16 full-length feature films.

• Cost: In 1911, when Eastman Kodak modified its exclusive contract with the MPPC, to allow Kodak to sell its raw film stock, which led the industry in quality and price, to unlicensed independents. The number of theaters exhibiting independent films grew by 33 percent within twelve months, to half of all houses.

Modern history:

• VHS: During the 20 years or so that VHS tapes were the primary medium for retail movie sales and rentals, few movies were released longer than two hours, since that was the capacity of a VHS T120 tape, played at standard speed. With DVDs now the medium of distribution, that limit no longer applies. (Source: Comment to question.)