The amount of time it will take to leave your system is very dependent on the individual, but your chances are better if you are a) Male b) low fat-to-muscle ratio c) naturally a fast-metabolizer.
Don’t exercise two days before the test (increases excretion of detectable metabolites), or drink too much water on the day of the test (because you can show up as a ‘diluted’ sample aka automatic fail or re-test).
Most of all, don’t worry – more stress will make you more self conscious, make your health worse and make it harder to stay clean.
For regular smokers (people who have been smoking every day for several months or more) marijuana takes anywhere from 2 to 6 weeks to leave your system, or a month on average.
If you used every day for just one month, and if you are really very small and skinny, then 12 days should, hopefully, be enough time. However, predicting how long marijuana will take to leave your system is not an exact science. Ideally, you should wait a month before taking the test, to be safe. But if you cannot postpone the test, then, all you can do is not use, and hope.
Again, I would think that 12 days would be enough time for you. But if you asked me, “How long will weed take to leave my system in my particular situation?” I would have answered, “Around 2 to 3 weeks, maybe a month at the absolute, absolute most.” Twelve days is kind of the minimum estimate.
I passed a drug test after only 9 days of being clean (I would’ve waited longer, but I couldn’t keep stalling and postponing the test), but I also drank Herbal Clean. I am also short and skinny. So, I’m not really sure if I passed because I was clean, or because of the Herbal Clean (or a combination of those two factors). But at the time, I had been smoking pot at least once a day for four years.