How much water do you have to drink to flush THC out of my system?

My uncle was on probation and had a drug test every Friday, for like over a year and he smoked every single day and still past every drug test. The day before his drug test, he drank nothing but water all day long and only failed one time in over a year. That means it does have to do with how much water. He's 260 pounds and 6'2 over weight and still did it.

Actually, drinking water only purifies the system and allows you to disguise the thc in your urine in an organic and untraceable way, a lot like clean drinks. ONLY drinking water will wipe your system out for a period of time, but once you stop drinking and your urine begins to turn a thick yellow color and is no longer clear, then 9 times out of 10, you wont pass the drug test, the user (or cheater in this case) must drink water and do some kind of exercise to burn your fat cells and sweat out the THC as well. So it doesn't really matter how much you drink, your urine just needs to be diluted. The only easy and 100% effective way to pass a urine test is to refrain from use.