It all depends on the cut off point in the specific test. Employment tests are a bit different from probation tests---which are a bit different from the surprisingly strict (IMO) very low cut off point that occurred in the case of Josh Gordon.
Bottom line, I have passed tests in a bit over a week (8 days) while being a chronic weed smoker back in the day 10-12 yrs ago, but I would think that something around 15-20 days would be sufficient just to be safe. There are rare cases that show up 25 days later, even up to 30 days--but it is very rare.
For a one time use, he would most likely be alright after 10 days, maybe 14 at the most--unless he's significantly large with a good amount of fatty tissue. Again, I'd err on the side of caution though, and try and throw in an extra week just for good measure if possible.