A long time. One hit of pot lasts up to 6 weeks in the blood at significant levels. Marijuana is fat soluble and sticks around in the fats cells and fatty tissues like the brain as well as the liver for many weeks if not years. August 2013 american journal of psychiatry has a lead article on the use of n-acetyl-cysteine in marijuana withdrawal. Nac may be helpful. It also helps bipolar depresssion.