If you do nothing to remove THC from your system, it can be detected in your urine for about one month. However, apparently they can find out if you have THC (or possibly other drugs) in your system by doing a test on your hair follicles.
Almost anyplace that requires a drug test has procedures in place is someone tests positive. Basically, if you test positive, you deny you've done any drugs and that there is an error in test and they are required to give you another test in within a certain time period. Usually that time period is long enough for the remaining traces of whatever drug to get out of your body, that is, of course, if you stop doing the drug. So if you have a postive and then a negative test, they'll hire you (or not fire you), but might have you take radom tests for a period of time. These are general rules set-up which most companies adhere to, but each campany may have slightly different criteria for their testing procedures. I just went through all this a few months ago when I got hired as a Federal employee and they laid all that out. I didn't have any problem as I hadn't done any drugs for well over five years (pot was always my drug of choice, but I gave that up when my son was born).