|Regarding how long it takes for THC to leave your body:
First it depends on how often and how much you actually smoke pot. Then it depends on your body weight, fat percentage and what your metabolism is.
THC is stored in fat cells, so you can only estimate how long it will take to be metabolized out of your system. I ran into a good website that says to estimate how long it will take for THC to leave your system naturally take your weight and divide it by 3, that will give you an approximate date for a daily user.
However, it does not specify for a daily user how much you would actually be smoking. Another site states it can take anywhere from a few days to up to 11 weeks (I think the later is more of a concern for blood tests). Hair tests... well, it is going to stay in your hair for as long as you have it... if you stopped smoking 3 months ago, the only the new growth after THC is gone will be clean, the rest of that strand of hair will have it... So, if you grow an inch of hair in a month, it took 1 month for your system to be cleansed of THC, of that 3 inches of growth, only the 2 inches closest to your head will be clean. If your hair would be 4 inches long, the last 2 inches will still have THC in it.
If you don't smoke frequently, THC leaves the body quicker... I have passed a test after only about 6 days, but at the time, I had only smoked about once in every 2 months.
As for taking things to mask tests... be very wary about this... as time goes by, companies update their tests to include testing for masking agents. And, if you test positive for a masking agent... then they will automatically fail you. I read above someone suggested chlorine, bad idea, it can be smelled even before you test it. Also, the first time I ever took a urine test, there was an attendant at the door, the door had to remain open. So if that's the case, it won't work at all. Same if you use someone else's urine. By the way... if you're going to attempt that, MEN NEVER USE WOMENS URINE!!! Its a bad idea, hormones alone can give that away. Much less all those stories you hear about the woman being pregnant.
There are other products that you can use, but if it's something that is only going to mask the fact that you use, again, be very wary... companies are constantly updating their tests for these products and if you are a very heavy user, you are taking a risk that they will not work regardless of guarantees. I know people that have failed using "guaranteed" methods. There are also sights available that claim to remove THC from your system and not just mask it, but believe it or not, they take about as long as abstaining from it anyway. So, if you want to get a better job or stay out of jail, you really are better off completely abstaining from it.
The state that I live in requires that all failed drug tests are reported. They also put this on your driving record so for the rest of your life it's listed that you have used illegal drugs. Besides the fact that you have now risked fines and imprisonment for using pot. But, hey, alcohol is legal right.