If your system was completely clean and that was all you had, plus considering you sound thin, only a few days. THC can stay in your body for up to a montjh or so, but that takes time to store up that much. Starve yourself for a couple days and do some cardio, that way youll burn into fat reserves, but again, i wouldnt really sweat it. I'm a little overweight and would be able to pass a UA after 3-4 days after smoking one joint, considering I was completely clean before that. Just stop smoking it if you are worried about it.