a piece of work is finished when you say it's finished. When it does what you wanted it to do, or when it becomes clear that it never will do that, and you just want it to be done so you can move on.

When a piece is done it will probably be: unified (there isn't anything that sticks out as underdeveloped or messy when it was supposed to blend in), balanced (nothing makes one side of the piece feel like it's going to tip over on that side its so much heavier over there. of course not all art is supposed to please the eye, so this one doesn't always apply), proportionate (nothing is out of whack, it looks like it is supposed to. Even if there is a lot of abstraction, proportion is important), contrast (the lights and darks compliment eachother well, creating the illusion of depth or not, shape or not...).

You can't force a piece of art to be done. My guess is that you've been working on this piece for three years because you, well, don't really work on it. If you feel stuck, have someone else look at it. Sometimes all it takes is making something an 1/8th of an inch wider or smaller, darkening something just one shade. Post pictures!

Dali is the man, I have a few of his works around my room.

Doing a Spanish project on Dali over the break. The man was truly brilliant. Eccentric, but brilliant. Dali and Kandinsky are my two favorite artists.
I believe there was a Kandinsky quote relating to this subject.
Let me find it...

"The true work of art is born from the Artist: a mysterious, enigmatic, and mystical creation. It detaches itself from him, it acquires an autonomous life, becomes a personality, an independent subject, animated with a spiritual breath, the living subject of a real existence of being." (Wassily Kandinsky)

Not the exact one I was looking for. But still good.