What determines the duration of a scene (movie)? Is it the animation itself or the dialogues in the movie? How can the movie director tell how long the movie will be?
This is a very common question, and the answer is that the length of any scene in a film is usually determined by a combination of the storytelling demands of that scene and the style of the project. Generally, it is the Director and his Editor who work together to decide how long the sequences (a sequence, or "scene," actually being made up of a lot of shorter "shots." A "shot" would be from a cut to a cut) should be.
The goal for the Director is for the scene to be long enough to tell the story he needs to be told in that scene, but short enough that it doesn't get boring or start to feel aimless.
As animators, we are usually given a predetermined frame-range to work with, so those decisions are usually made before the animator starts a scene. Every once in a while, though, a chance might come up for the animator to affect the length of the scene. Sometimes you might create a really cool action that will change the Director's original idea for the shot, and he might add more frames for you to work with, for example. But usually, they'll tell you that you have a certain number of frames to work with, and part of your job as the animator is to find creative solutions that will make your animation ideas work in that exact number of frames.