When to stop

I can never tell if I prefer a line drawing or inked. When do you stop? Sometimes I'll leave a painting unfinished for a year then come back to it while working on other things and the additions or subtractions are perfect and the painting just feels done and my brain can stop thinking about it. I wonder what it would be like to work on one piece only for your whole life painting over it scratching drawing clearing repainting. I love the concept of the opus 40 by Harvey Fite. http://www.opus40.org/ if you haven't visited, you must.