There's story I heard once about someone who went to an exhibition of primary school art. One class of tots had very impressive paintings displayed - they were naive but amazingly effective.
The guy remarked to the friend who had invited him that the teacher of this class must be really gifted to get this kind of work from such small kids. The friend replied, "No, she just knows when to take the pictures away from them".
This truism can be related to almost anything. Do you know you've had enough to eat, when you've worked enough on a presentation, when you've hung enough pictures on your living room wall? There's a fine line in most projects between something being done and it being overdone.
And screenwriting is no exception. There's usually a moment in most writing group sessions when the person whose work is being read has had enough feedback. If they get any more, people will start making mad suggestions and frying the victim's brain ("maybe the main character should be an alien?" That kind of thing).
And there's usually a point for most scripts where it's been tweaked and rewritten enough, where any further efforts will disimprove it, flick a bit too forward on the dial and tip your work towards a producer's waste paper basket. And sometimes you can't see that point until it's happened.
You have to be vigilant, to know when your screenplay is working and when it isn't. You have to know more than anyone else... when to stop.