A few things ---
The ONLY way to get THC out of your system is time. It's in your blood, hair, and stored in fat cells in your liver and kidneys and such. There's some masking agents that will work on some tests, depending on how specific those tests are designed to be, but in some cases, they also look for those masking agents and that can result in what is essentially a failed test.
If you smoke a lot like I do, (multiple times a day every day), then you probably won't be clean for around 2 months.
If you are more of a part-time, social-smoker kinda guy, that's gonna be about 10-20 days.
If you just took a couple hits a few days ago and hadn't smoked in months before that, you'll probably be clean by the weekend.
Everybody is a little bit different, so all of those are rough estimates. Your mileage may vary.
I get tested twice a year like clockwork, and I've passed a test after smoking a joint in the parking lot. Here's how :
Get some clean piss from a friend that doesn't do anything they will be looking for.
Put that in a little baggy or something that won't leak.
Tape it to the inside of your thigh as high up into the grundle as you can tolerate, the higher the better. (You can't submit some room-temperature urine and expect to get away with it)
Get into a stall and put the contents into the test cup.
I've passed 6 straight tests with that strategy. It's not exactly a day at the beach, but it's better than getting fired, and it's better than not smoking.
Sometimes it takes a crazy person to see the truth. If so, I'm a freaking lunatic.