hmmm- interesting
i find that knowing when to stop is an important key

I don't think a painting is ever finished. i think you just get to a point where you are afraid to fuck-up what you have done already, and just put down the brush.

I think the answer to your question will come from looking within and what lies behind your question in the first place.

There are so many different views, theories, criticisims, and accolades,etc. in the art world.

I don't believe anyone can you give you a just or correct answer except for yourself, and until you find confidence in your skill, purpose, and intent in your particular style of expression you will continue to wonder the answer to this same question.

There is much to be said of the many comments you have already received and though not wrong...not necessarily right either as your question is not defined well enough to offer said advice.

If you are, for example, looking to create, exhibit, and sell your artwork...then, that of course would then take looking at your niche and the particular attention the public in your venue gives to such artwork.

Personally, my art "just happens" to me...I do not go into the experience with any defined idea, sketches, or intent other than pure, free flowing expression. I am inspired to pull out my brushes, paint, and canvases and just go to it. Then at some point in the creative process on canvas, I will stop and realize "I am done, that's it!" ( or at times..yes, it happens to us all...I will not have that "a-ha" moment, will become frustrated, and will discontinue paitnting a piece and let it sit until I am ready to return to it at a later point to complete it. )

And, as for pieces being "finished..." As much as I may find that "I am done" on canvas...the creative process, the experience does not finish then or when the paint has set. I am always learning from my pieces. Likewise....when you allow others to view your work whether in private or public...they seem to take on an entirely new life and being as the piece is opened up to the experiences and connections others see, seek, or wonder of your work. Wonderful, amazing experiences!

Then, also, I have had some that I created that although I had the feeling it was done...the piece was disturbing for some reason. Then, days, weeks, months, and in one case a year later...symbolic images later revealed themselves of an inner knowing of the expression that had occurred yet was not fully realized in my life at the time - to say little more, if when I die people find and enjoy and exalt my work....some lucky someone out there will have that original piece to share with the world as I had to cleanse my house of it's energy.

Hope that's some food for thought for ya...just one perspective of many.
Till next time!