I've wondered this also. Basically as the first post says the results of how long are very varied. Basically I have settled on 24 hours being a safe bet and anything before still registering.
What I am curious on though is if you are swabbed whilst driving (we have plenty of drug tests in Aus, dunno bout elsewhere) is there a threshold where they determine you have trace amounts of THC but not enough to effect your judgement?
-Trippy