"When I've painted a woman's bottom so that I want to touch it, the painting is finished." - Pierre-Auguste Renoir
When not painting bottoms, it's often hard to know when a painting is finished. Sometimes, you just know and you put your brushes down. It's a quiet but wonderful feeling of satisfaction.
Sometimes you're unsure. When this happens to me, I try to put my brushes down and let a painting sit and my feelings marinate. I'm not always successful at stopping when I should, but stepping away from a painting, and coming back to it is one of the best ways to figure out what bothers you about it. Too much red? Not enough light? Sometimes I can put a painting away for a week, and then when I look at it again, I realize I love where I stopped and I know it's done. Other times, I pause to look at a painting everyday for weeks. I turn it around, look at it upside down, and wait. If things keep bothering me, it's not finished, and I keep painting.