IMO unless you smoke all the time it won't take 30 days. If you took one hit, and that was it, for like the last 6 mos, IMO it will be out of your system in detectable levels in a few days. The 30 day thing is for people who smoke regularly.
But if they fire for any trace you could be in trouble. Usually someone who takes one hit registers so low on the nanograms they won't fire for it. Different companies set different standards they ask for. If they ask to test down below 10 nanograms or something crazy you could be in trouble. If it's like 50 nanograms though I bet you are ok.