Depends a lot on your weight too. When I was in the Army, I was 5'10" and weighed 100 pounds even. Not much body fat there. I never failed a test. They were pretty damn sure I smoked pot, but because I never failed a test, not much they could do. They were so sure that when someone in the barracks was smoking pot, and the smell went through the ventilator to my captain's office on the first floor, he came straight to my room. (I was just reading, it was someone on the third floor, and they got busted.) When my captain told me I was going back to Grenada a second time, the way he told me was hilarious. I was on sergeant-of-the-guard at the desk and as he walked out the door, he said offhandedly, "Okay, specialist, you can go on back down to Grenada and smoke some more of that pot." And then he was gone, he left me no time to even reply. But my mouth was hanging open and I was speechless anyway.
A funnier piss test was in Grenada. They piss-tested us, and I'd smoked so much pot there, and was so absolutely positive I was going to fail, that I went into the wooden box we'd built as a bathroom, and toked on a spliff WHILE I pissed in the jar. But the tester dude saw we were all paranoid and freaking out, so he told us to calm down, he was going to leave the samples baking in the sun all day, and we'd all come up negative. And we did. (What a cool dude he was!) I might've passed anyway, because of my weight, but I don't know -- pot was $120 per POUND there, and I was toking down a good 15 or 20 joints a day. Shared with others, of course.