How do you know when a painting is finished?

How do you know when a painting is finished, thats what I have been asking myself recently. I'm not the first and won't be the last. I am talking about painters who keep adding one-last-thing to their work and the painting seems endless and never finished.

This one was finished for several days, in fact you can see the earlier version in a previous days blog. The trouble is I keep adding a new cloud here or there, some new mist here, how about making those hills more prominent? I will have relief from this torture very soon as my friend will be taking it away for valuation in advance of the auction (charity).

I heard about one famous painter, I forget the name, who actually went around to clients houses months after the painting was finished to make further changes. I promised not to do that! For me once it leaves me then thats it bye-bye picture.

I've got to go now and just make those clouds a bit fluffier in the distance .......