Hey trude,
It does usually take about 28-30 days to leave your system but I have also heard that
marijuana stores its self in body fat so if you lose a few lbs after you have been clean for 30 days it could show back up. Your best bet, if you need a clean urine is to to stop smokin and just to be on the safe side, go to GNC or another health food store and get something to clean, detox your system... if you ask someone at the counter they will usually tell you what to take.