"When to stop? " can be applied to many moments in life. But I'm focusing now on:
When to stop working on a painting or other piece of art? In other words, When is it finished?
The short answer - a work of art is never finished until it leaves the artist's studio and sometimes not even then. I recall reading that the abstract painter Clifford Still was sued by the owner of one of his large canvas paintings. Years after it sold for big money, it was returned, damaged, to Still, rolled up, with a note essentially saying, "Fix it". About a year later Still sent it back. Still had completely painted over the original work. The Judge sided with Still, since the owner didn't specify how to fix it.
Sometimes the answer to "When to stop?" is clear: a quick sketch that sings to you, or when you exhaust an overworked piece to further one's knowledge.
But often you reach a stage in a work where it seems unified, but the question arises, should I leave it now, or continue on? You can always do more, but recall the late comic Gilda Radner's adage, "There's always something."
It may be worth while to reframe the question from, "When to stop?" to "Should I continue?"
My advice: 1. Stop work and sleep on it. Your creative brain (right side) has probably reached its limit and has turned things over to the analytic (left side) of your brain which is posing the question. 2.
When you get up in the morning to study your work, your right brain will be in control again. If there is somewhere creative to go with that piece, then proceed. If not, sign and date it and start something new.
The photo above is of a large Matisse masterpiece at the Pompidou Museum in Paris. Notice the shaded and scuffed area behind the sitter's head. Matisse must have reworked the head many times, but wisely left the resulting background alone when the portrait was complete.