Depends on the type of drug test and how often you smoke. If it's a urine test, typically a month or so if you're a frequent user. A hair test can go up to 3 months back unless you have 18" of hair which then can go back 180-360 days. If you shave your head, they're then required to take body hair or a blood test. I'm unsure how long a blood test dates back.
