For me it's when I have at least 90% confidence that I can do better. It's never really about what you dislike about the person, it's about the probability that you can walk back into the dating market and be assured that you will pull a better deal rather quickly. There is no point in looking for a minor improvement, it's just not worth redoing all the time and effort it takes to connect just to get a minor upgrade. So you have to be certain that you're working with absolute awful conditions that you know you're not going to have trouble pulling better before you leave what you've already worked for.