Got this info off answers.com......
According to the website of Columbia University's Health Services:
Marijuana itself does not "stay" in your system, but the main chemical in marijuana, THC, does. It fact, it may stay in your system forever because it is stored in fatty tissues of various organs.
Despite being stored in body fat, THC is only detectable for a certain amount of time. Many drug tests, including urine tests, look for 9-carboxy-THC which is the by-product of your body metabolizing the THC in your fat.
Read more:
http://wiki.answers.com/Q/How_long_does_marijuana_stay_in_your_system#ixzz1OWlaWqhS