I think it's pretty difficult and relies on you knowing yourself and being willing to push yourself. I think a good rule of thumb is if you've completed three pomodoros (or equivalent) in a row, you can justifiably take a 30 minute break (and then go and do three more pomodoros).

I know where my burnout point is only because I once pushed myself until I burnt out. Though I may still revisit that point again in the future and try to go past it.