I can't give an exact answer, but I can tell you that it happens gradually. Initially you feel great, and you're incredibly productive. Two months later you're taking the same amount, but it doesn't give you that same feeling so you increase the dose a little bit. This repeats for awhile until you start noticing side effects and try to stop. That's when you're stuck - you can't do the work you used to do unless you're medicated, and you can't stop without putting your life on hold for a very long time.