single use is nothing to worry about. that will be out of your system within a week, tops, if you are indeed clean before you smoke.
check this out:

Erowid Experience Vaults: Cannabis Urine Testing - THC Detection Times in Occasional Smoker - 37341
Yam Reviewed by Yam on . Could use some advice for passing a DT Ok here is the story, I am on 12 month probation (first time offense, judge was fucked up), been clean for about 49 days now (since 10/15) passed my first drug (11/14) only 28 days after i smoked naturally. My second DT (12/18) is 13 days away, which gave me over 30 days to pass it naturally. (i havent smoked) My question is after i take my second DT how long do you guys think it will take me to be clean for the next one ? Some info on myself: Weight: 165 Height: 6'0ft Age: 20 Keep in Rating: 5