Have you run out of ideas?
One of the simplest ways to tell if your painting's finished is if you've run out of ideas. Paintings are visual displays of your creative talent and ideas. If you can't think of anything else to add to your painting, it's time to move on to another project, to put it simply. If you're stuck for ideas, the best thing to do is to accept that your work is complete. If you don't accept your work is complete, you could end up adding things to your work that aren't really necessary; in other words, you could be coming up with ideas that aren't so good and adding them to your work just for the sake of it. Once you've run out of ideas, don't be tempted to go back and add more to your work, instead just leave it as it is.
Are you bored with your painting?
Artists put a lot of passion and dedication into their works. Paintings can take a long time to complete and it's the artists' motivation that keeps them painting. However, it's perfectly possible to become bored with a painting, even if you started off being completely passionate about it. Sometimes as you get into completing a painting, the enthusiasm goes down. If you find yourself less enthusiastic about your painting - if you find yourself bored with it, let's say - then you should consider it finished, for now at least. Move on to something else - there's no point working on something if you're not passionate about it.
Does it look finished?
Try to see your painting from another person's perspective. Would they consider it finished? If you're unsure whether your work is complete or not, simply ask other people what they think. If they don't think it's complete, ask them what's missing from the painting. If, on the other hand, they do think it's complete, then as long as you're happy with it as it is, you've got a finished painting on your hands. Of course, deciding whether your painting's complete is ultimately up to you and you alone, though it can be helpful to get opinions from other people. When deciding on whether your painting looks finished, ask yourself these questions: does it have a clear subject matter? Are the shades and tones done effectively? Do the colours match the tone of the painting? Does everything tie in with the message I want viewers to pick up on?
Revisiting a painting
Remember there's no hard and fast rule that once you leave a painting, you can't go back to it. Loads of artists regularly put works to one side for a while and revisit them at a later stage. You may well consider a painting finished, only to revisit it a few months or so later to add some extra details you think would enhance its appeal.