That stuff works but its really important to use it correctly. Call the number on the bottle if the directions are vague, they'll tell you how many ounces of eater to drink before and after, plus tips like stay away from milk and cheese, plus coffee and some some other stuff for like 24 hours before you take the test. DO NOT chug way too much water right before the test if you use the drink, you can flush out all of the creatine and b-vitamins before they can bond with the metabolytes in your system that you want masked.
If you don't use the drink, I'm a full believer in water and exercise, but you want to stop exercising about 48 hours before the test if her a big fella. Burning calories will start to burn some of your fat, and can release THC that has bonded with your fat, and thus more will be in your pee. But if you can burn enough calories the week before, then flush your system with water you have a better chance.
If you don't use the drink, you want to be POUNDING water the day you go. Buy some azo pills (make sure its the pain relief, not just the cranberry pills) and take one an hour orntwp before your piss test; the stuff usually makes your pee dark orange, but all the water will dilute it and your clear pee will look nornal--you might want to practice with this stuff to make sure you get your timing right. Sometimes you'll take one and then the first pee is clear, the second pee is orange, the third pee is perfect and the 4th pee is clear again. Its a little different for everybody but they won't make you take it again saying your urine is too diluted.
Good luck brother. I've been taking pee tests monthly for five years and I think 20 days is plenty- I don't care how hefty someone is. My best record from going from smoking daily to clean is about 6 days, but I'm 6'1" and about 185, probably about 8-10% body fat.