I have a simple rule for quitting jobs, and it has served me pretty well in the past.
You should quit your job when you stop loving your job. Sounds straightforward, but how do you define that?
Well, I believe the definition of whether you love your job, is whether you look forward to Mondays. If too many weeks go by without this feeling, you should start evaluating your situation and working out if you could be happier somewhere else. Don't settle. Life's too short.
The other aspect to enjoying your job is learning. As soon as you stop learning, resign and move on. Working with highly intelligent people who are constantly teaching you things is a crucial part of of self development.