Here's a link to the most realistic chart I've seen on how long certain drugs stay in your system. http://www.friendslab.com/index.html?drugdete.html&2
Cool link, RMJL. How long it stays really depends on a few factors: How often and for how long have you smoked? How good was the shit you smoked (a new one I've added lol)? How fast is your metabolism? How much body fat do you have? Those factors can leave you with a "clean out" time of between a day to over 100 days. So.... there is no quick answer to that question. If you plan to fool a drug test, substitution is the safest way to go about it. (Do a search for "Urine substitution" at any search engine (well... probably not the dea website engine if they have one)).