I drug test people all the time at work - if you smoke occaisionally (and I do mean the odd puff), it takes up to 7 days to clear the system when being tested in urine. However, if you are a pothead and smoking most days, it will take up to 28 (as it accumulates in your system). So be sure of what you are doing before you pee as tests are very sensitive these days - excuses won't wash as they will have heard it all before. If they go to the trouble (and extreme expense) of testing hair, I think cannabis and other substances may even stay detectable in that until it is cut off or disposed of.