Everything differs from person to person, depending on how fast your metabolism is and how frequently you smoke. 2-6 weeks, 4 being normal I guess. How does it give MJ a bad rap? I know advertisements a couple years back had some strange statistic, like 50% of all people involved in fatal car crashes tested positive for marijuana. It did NOT say these people were under the influence, but that's what many who saw the commercial were led to believe. The bus driver might have been clean as a whistle during the day, and only partaken in a little puff puff at night. Does not mean he was putting the kids at risk, just that he had smoked pot (maybe even only ONCE) in the weeks leading up to the test.