Depends entirely on the amount you smoke and how often.

For instance, if he smokes 1 hit and he never smokes again. He's looking at a few days.

If he does it all the time, upwards of a month.

That's why they make the wizzer. It's literally a dildo that is attached to a bag of piss. That way you can "pee" in front of the doctor with someone else's urine (clean of course) and still smoke a lot and not get caught.