how long should i need? maybe someone has some input

i will have to take a drug test for my job. i got it on tuesday the 13th and my boss told me i have something like 15 days to take it, no rush. i smoke maybe once or twice a day, not potent stuff, every day for years now. i weigh about 130 and am about 6'1...so you can picture a tall skinny guy and, given the height and weight, you can imagine i don't have a lot of fat on me. i smoked that night and haven't smoked since. i've been drinking Lipton green teen, the orange and mango flavor, and have been drinking a decent amount of water every day. how long do you think i need in order to pass? i want to give myself as much of the allowed time as possible but i also want to start chiefing as soon as possible. anyone with some knowledge?

p.s., if i fail they call right? what if i pass? will i only know if i still have a job a week later or what? cause i would hate to pass it and still have to wait a week to start smoking again because i dont know if i passed or not