For me the most relevant question and perhaps the only one is, "When are you finished?"
When do you stop? Or rather, why stop at all? But you have to rest somewhere. Of course you can stay on the surface all your life, Balzac's Frenhofer. And all of your life's work can be seen as a picture -- but that is merely "true" There are places where you pause.
Thus it might be argued that when a painting is "finished," it is a compromise. But the conditions under which the compromise is made are what matters. Decisions to settle anywhere are intolerable. But you begin to feel as you go on working that unless painting proves its right to exist by being critical and self-judging, it has no reason to exist at all -- or is not even possible.