I set the timer and try to get to 5 minutes. Ususally I don't go much longer than that. Gets too thin and yucky. I will start fresh and go another 5 sometimes.
I personally don't think that going longer would be healthy. Someone posted concerns about the toxins re-entering the system. I'd rather spit and start over than possibly take back toxins. The oil isn't that expensive. Good Luck! Do what you feel is best for you. Track your results and go from there.