It can be stored in cells for months after last hit. They just have to nap a hair out of your skull and they'll find out. THC do not loosen up in water, did you ever try to cook hash? Nothing happens. But when you cook it in oil, it loosen up and mix with the fat. THC is stored in fat, DNA etc. etc... However , the effect does not last that long, (something you probably knew allready). If you're going to be tested, drink one of them cheech & chong drinks. Makes you pass...
-metabolism
-whether or not you are a frequent herb smoker.
-etc
but a good guesstimate would be anywhere from 7- 30 days for a piss test.
i dont really think water actually flushes it out of your system but if you dont have much in your system and you drink a lot of water before a test it will dilute your urine and allow you to pass.