I am 400 pounds and rarely smoke marijuana. (once every 6 months) how long will it take for my system to be clear?

Depends on how much: you smoke every six months. No matter what you weigh, a single use of marijuana will not give positive test results for more than 2-3 days at most, if the amount smoked is modest. If you are smoking larger quantities for the entire day, it may take several weeks to clear, because of the high fat solubility of THC. But a single joint or bowl once every 6 months should, again, be 48-72 hours.