I am 400 pounds and rarely smoke marijuana. How long will it take for my system to be clear?

Cannot say. Depends on test item to be used (hair/nails longest), and what "rarely" means to you. MJ redistributes in body fat and then leeches out, so urine 'can' be positive for 5-90 days for regular users. You may be towards high end due to wt. Sounds like time to make responsible decision as to priorities--MJ or job/court/etc. Good luck.