About Scene Length
The length of your scene is dependent on how many drawings you need to expose and for how long. A drawing can be a held for several frames to make the action appear slower or for just a few frames to make the action appear faster. When setting the timing for an animation, a lot of adding and removing of frames occur to get the timing just right. This invariably changes the scene length.
Adding and removing frames can be done in the Timeline, Xsheet or though the Scene menu.