There are different types of testing equipment for urine testing. I used to buy drinks and wait an hour, and you are good for a few hours. Those worked 10 years ago. My girlfriend took a drug test recently for a job at a big-name company, after a month of not smoking, and drinking cranberry juice, green tea, and lots of water all month. and exercise all month, and she used the bottle of stuff supposed to clear it out (i forget brand) They used a gas spectrum analyzer, and she FAILED. They said they know she smoked pot. (they still offered her the job, as long as she enrolled in a rehab. fuck that)
We got one of those fake piss bladders that you strap to your leg. never failed again.
I think it depends on how much you smoke, your metabolism, and what type of epuipment they use.