I think Cav's idea that only a moment can be perfect is pretty swell. There've been a lot of interesting thought & quotes...
Here's a moment that might be perfect. Picture this. An artist has been painting for hours this day and for many days before. The idea has percolated and the dimensions seem right, the piece is saturated with his spirit. Is that bit of green there too blue? No, is it green enough, good enough. The work is complete.
When did the artist know? There has to be a moment when there comes a satisfaction of the work from within, assuming the artist is still feeling honest with himself and is (ahem) not in a hurry to get a show together.
We once bought a small piece and afterwards the artist asked to come in and add something that he'd planned on doing, but hadn't gotten around to it before it left his studio. We liked it both before and after the extremely minor addition, but it bothered him.