It's very hard for me to know when a picture is finished. It's mainly the fact that I get tired. Every time I start a fractal, I have a very good idea of what I want to do, and the hardest part of the work is getting that (choosing/writing a formula for the first layer is the toughest step!). Since I'm always aiming for this "perfect" picture in my head, I never get to it, so basically there comes a point when I say "this as good as I'll manage"