How do you know when to quit?

How do you know you're "done"?!  I get asked that all the time.  As the painting progresses, I slow down more & more....that's the first sign.  Would another stroke help?  Then, I snap a picture of it and make it my screensaver.  I spend some time with it this way...and darned if I don't see just one or two more things that I never saw in my studio (the smaller size matters!).  Back upstairs I go and execute what I saw in my head.  One more photo, and I repeat the process till there are no more "aha" moments to be had.  Then...it's done.