The speed at which marijuana leaves your body depends both on the speed of your metabolism, as well as on the half life of THC. Unlike most other substances, the half-life of THC can vary quite a bit, depending on who is using it, when it's used, and several other factors. For example, it's estimated that THC can have a half life that ranges anywhere from 1-10 days. It's almost impossible to find out what that half life is, which means that it is very difficult to use it to calculate how long it will take for all of the THC to leave your body. Not only that, but your metabolism and how much marijuana you smoked will also have an effect on the length of time that it will take you before you can test without testing positive for marijuana use. After all, the half-life is only the length of time that it takes for you to lose half of the THC that is in your system. If you've smoked a lot of marijuana, then it will take you longer before the THC in your body drops back below the amount that is detectable by most drug tests. For this reason, when you ask "How long does marijuana stay in your system?" you have to realize that it's very unlikely that you'll be able to get a usable answer. The most common period of time is anywhere from 3-30 days, depending on your metabolism and the amount of marijuana that you've smoked. This is unfortunate, since it means that while most drug tests can only turn up evidence of other drugs if you've been taking the drug within a few days of the drug test, marijuana could still be detected in a urine test up to a month after the last time it is used. Sorry, no easy answer

Not likely, because the chemicals from pot store themselves in your bodyfat (even if you are skinny) it takes about 30 days for it to clear your system. And if you have smoked large amounts of pot in the past, you may always test positive.

if you're really worried, drink a bunch of water right before you go in... and then drink a glass of orange juice (to give a little color to your pee)

Probably the best thing you can do is dont take the test, or, bend over, place your head between your legs, and kiss your sweet behind goodbye!