The film is divided into reels. The reels are usually equal in length, on an average from 900 to 1,200 feet long. The combination of the reels forms the picture. The usual length of a picture should not be more than from 6,500 to 7,500 feet. This length, as yet, involves no unnecessary exhaustion of the spectator. The film is usually divided into from six to eight reels. It should be noted here, as a practical hint, that the average length of a piece (remember the editing of scenes) is from 6 to 10 feet, and consequently from 100 to 150 pieces go to a reel. By orientating himself on these figures, the scenarist can visualise how much material can be fitted into the scenario.
Astonishing, in a way, to see guidance so concretely dependent on the particular technology, given the abstract nature of most of this. Of course he really means duration, time, but it's like saying "a film should be 65 to 75 gigabytes, as not to exhaust the viewer".