I think you're right - the longer the stay in a job the greater the chance you'll both settle (trading aspirations for comfort) and the more you'll lose faith in your own ability to find a new one.

My own rule of thumb, and I'm no shining example, is once you stop being challenged - no matter how nice the coffee, comfortable the chair, or hilarious the co-workers - is to move on and find something that tests you. Something that keeps you on your toes.