Buy urine drug tests on Amazon. I've bought this 10-pack there for $7.75 twice now: [ame]http://www.amazon.com/INSTANT-Single-Panel-Drug-Test/dp/B004C37VV2/ref=sr_1_4?ie=UTF8&qid=1414872929&sr=8-4&keywords=urine+drug+tests[/ame]
I'm a daily smoker, and it took 35 days of no marijuana, plus a lot of exercise and sweating, before I tested clean. It took my wife only 14 days. We're both in very good physical shape, but I weigh 200 lbs - near twice as much as she does.
I must respectfully disagree with the people here who have suggested synthetic urine or drug masking agents. Modern labs can bust those easy as pie. Now maybe the lab you'll be tested at is not a modern lab with the latest techniques. I don't know. But if they are, you'll get busted. The last pee test I took was rejected because they said I had too much liquid in my pee, and not enough particulate for an accurate test. The lab was a new, state-of-the-art lab with sophisticated drug tests. I was re-tested the next day and passed. But if they detect making agents or synthetic urine, you'll fail and they will not re-test you.
The only way to be assured of testing clean in this day of modern science is to use clean pee - yours, or someone else's. But of course, you have to worry about temperature of the pee if you use someone else's. And you have to worry about whether they recently used drugs.
Good luck.