"Well, first of all, screenwriting isn't really writing: it's really part of the oral tradition and it has a lot more to do with the day your uncle went hunting and the dog went crazy and the bird got away than it does with literature. One of the indispensable ways of of judging whether an idea will work as a film story is oral presentation - you have to tell your story to someone.
When you first get an idea, maybe its five minutes long, then the more you tell it, the more you elaborate on it and the longer it grows. When the story gets up to about forty-five minutes in length and is still holding your listener's attention, so that you know that if you walk out of the room they'll follow you to ask what happens next, then you know you have something that will probably work on screen."
Paul Schrader (writer; Taxi Driver, Raging Bull, Affliction) taken from Schrader On Schrader published by Faber and Faber.