It helps to talk a little about my process first, which follows:
Spark of an idea.
Asking myself will this make a good painting.
Drawing on the Canvas.
Block in of forms and shadows.
Then repeated passes from back to front, light to dark, and periodic evaluation of the painting as a whole, i.e. "is this working?"
As the work progresses to a somewhat realized image I inevitably hit the "good enough" phase which can be translated to "I've finished the easy parts now it's time to slow way down and ask some difficult questions but can I get away without going any farther, the answer is usually no. So I ask myself;
Is the space convincing, does the painting feel like a fully realized three dimensional space?
Do the values work, and are they consistent?
Is the color, texture, detail enough or too much?
Do all of these elements support the focal point or story?
Then it's revisiting these same questions til it's done.
All of that said, the challenge is to keep asking the questions until I sit back and smile. This is my signal that I've taken it as far as I can and it's time to ask for a critique. Then fix the problems until my critiquers sit back and smile. There are some paintings that never reach that feeling of satisfaction and accomplishment. The more I study the more I realize those paintings are usually cursed by a bad beginning. The composition wasn't strong and or the idea was not fully realized are the two main problems.
The funny thing is the answer to the question of when is it finished changes as I evolve as an artist. Maybe I'll look at the question again next year.