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,...
Generally from 3 to 7 days, but up to 30 days or longer after very heavy use or for persons with high levels of body fat. (Marijuana does show up in hair samples up to 90 days later.)
It depends on the drug. An immunoassay for TCH, for example, may show traces for up to four weeks or longer, depending on the amounts used, weight of the user, and a couple of other factors. Others...