In fact, the time you'll test positive depends heavily on the amount you consume per time and the duration for which you have been consuming. I heard a lecture in forensic medicine about drugs and their 'testing behavior', where the professor (Prof. Madea of Bonn, Germany) told us, that they once tried to test this duration at the Institute of Forensic Medicine with a single long time chronic stoner. The experiment was discontinued after 100 days. If you just smoked once you can expect to be clean after some days, using some proved detox drinks, here I've read about some of them.