Stop when you have a good idea. Then you are set for the next day. Surprisingly in a month, the pages have added up. Barnes writes in long-hand for his secretary to type. That way the typed version is distanced, as though someone else wrote it-heightening his objctiviy.
Every cut makes it better.
Writing for movies is a craft job. Usually 7 to 35 more writers will be called in to rewrite the script. If you take it personally, you will be destroyed. Anything personal gets cut out by the studio because it is usually only meaningful to the writer. It is a profession.