Morning on Balboa Island, 9 x 12 inches, oil on linen
So...how do you know when to stop?, asked Kevin in the comments of my last post. The short answer is, When nothing in the painting bothers you.
Obviously, if you see an error, you have to fix it. It's like creating an important document–a resumé, say–and you spot a typo. You wouldn't leave that alone, would you?
One common tendency for intermediate painters is to put too much in the painting, thinking more detail somehow makes the painting better, more convincing, more realistic. But just as you wouldn't put everything you have ever accomplished in your resumé (including that time you won the beer bonging competition at the fraternity), trying to put everything into a painting is a ridiculous idea. You have to stop somewhere. But where?
In a resumé, you include only relevant information, as clearly and concisely as possible, and edit out everything that doesn't pertain to the job you're trying to get. If a piece of information isn't going to help you get that job, you don't put it in.
How do you know if a piece of information is relevant? You think about that job you're trying to get, and determine in that context, whether the information is relevant. You have to be clear about the purpose of this resume. You have to be clear about what you want to say, and say only the things that matter in this specific situation.
Painting is much the same. It's a form of communication, so you have to be clear about what it is you're trying to communicate. What do you want to say about this particular scene? That is your concept. Your job is to communicate that concept, and nothing more.
In my painting above, my concept was the mood created by the backlighting in the morning. I chose a boat to use as a focal point (one part of it, really) to achieve this. All the other boats were necessary to create the environment, but none of the details on these boats had anything to do with my concept. So I didn't paint any details. The reflection of the main boat on the water was, in actuality, very clear and visually seductive. I could have painted that, but again, that wasn't my concept, so I played it down as much as possible.
If you're clear about your concept, it will guide you in making decisions about what to include and what to edit out. You may have to put things in and take them out just to see whether they support your concept, and that may take a long time. In fact, I spent as much or more time doing that, as I do getting the painting to look like the scene in front of me.
When at last you're satisfied that nothing more can be added and nothing more can be taken out, and you can't spot any errors (you may be able to the next day, or three years from now, but for now, to the best of your ability, you can't spot any errors), and you've scrutinized every shape, every color, every value, and every edge to make sure they can't be improved by your current abilities, you're done.
Know what you want to say, say it, and nothing else.