Alright I've been researching the crap out of this and found a 155 page PDF on the NDCI site that is a study on cannabis detection times. Theres so much conflicting info I really don't know what to believe.

"At the 20 ng/mL cutoff concentration for the detection of cannabinoids in urine (using the currently available laboratory-based screening methods) it would be uncommon for an individual to produce a positive urine drug test result longer than 21 days after the last smoking episode.

For occasional marijuana use (or single event usage), at the 50 ng/mL cutoff level, it would be unusual for the detection of cannabinoids in urine to extend beyond 3-4 days following the smoking episode (using the currently available laboratory-based screening methods or the currently available on-site THC detection devices). At the 20 ng/mL cutoff for cannabinoids, positive urine drug test results for the single event marijuana use would not be expected to be longer than 7 days."

http://www.ndci.org/NDCIR%20VI.pdf