I'm not quite sure off the top of my head but I believe marijuana can stay present in the body for over 6 months. The THC can be found in fat cells years after smoking weed but as far as being able to flush the majority of your system your looking at half a year, give or take. Check out Lex's World, he has great videos on marijuana education. I believe somewhere in his archive he has a video explaining this topic.
The effects of smoking marijuana fade quickly, but the drug, known affectionately as "weed," can be detected in the body for weeks and sometimes longer. How long it remains in the system depends on how often or how much marijuana the user has been smoking.
Yeah like I said there is many factors but how much the person smokes is what matters. If its only 5% THC but he smokes an ounce in one sitting with 3 other people he isn't going to be clean for at least 30 days, and that is only if he doesn't smoke what so ever threw out that time. If he just takes a hit and doesn't smoke after he will be clean in literally 2 days.