Some screenwriters go by number of pages. Others set a time limit. One I know eats chocolate covered coffee beans all day and when his hand starts to shake that’s the time to wrap it up. There’s no right answer, except maybe the last one.
A lot of writers will hang in there until they finish a scene. They like that sense of closure. There is something to be said for that peace of mind. Especially if you’re neurotic and from what I hear, one or two writers are.
On the other hand, I like to stop when I get stuck. It’s usually in the middle of a scene. (Ernest Hemingway used to stop in the middle of a sentence. I always wondered if he wrote a suicide note that read: “I’m sorry, I just can’t go…”)
Instead of gutting it out, I will walk away from the computer and just let my subconscious mind work on solving the problem. I know that sounds very “Yanni” but for me, a relaxed state is when I do my best thinking. I’ll keep a notepad by the bed and in the morning just lie there like a slug and let my mind run free. Very often the answer will come. Similarly, the shower is a good place to problem solve, and one of the few shower activities you can admit to.
The added benefit of allowing yourself the night and morning to break through your problem is that when you sit down to start writing again you’ll already have a head start. You won’t have to face the “tyranny of the blank page.”
Writing is rewarding but never easy. We resist starting and constantly fight the temptation to stop. Kurt Vonnegut talked of the difficulty. He said whenever he’s in a room with writers they’ll all be bitching about how hard the process is. All except one. He’ll say it’s a breeze. Every day it just flows. Invariably HE’S the worst writer in the room.