How far does urine drug screen go back for marijuana use?

Marijuana takes a long time to leave your system. Much longer than any other drug that I know of. Most other drugs are out of your system in a matter of days, or maybe a week for extremely heavy, long-term users.

For regular smokers (people who smoke on a daily basis), marijuana takes anywhere from 2 to 6 weeks to leave your system. So, around a month is a safe bet.

For someone who is a more moderate user (who smokes only a couple of times a week, or several times a month) the length of time would be more like 10 to 30 days, I suppose.

Predicting how long marijuana will take to leave your system is not an exact science, so the best thing to do is to stop smoking for as long as you possibly can, and postpone the test for as long as you can without seeming suspicious.