You don't know the truth because nobody does. Everybody's different. Marijuana stays in people longer due to many different things, which I'm sure you've also read. So asking a question like that is almost impossible to answer. It's basically a judgement call that only you can make, because if you fail, you can't really blame anyone but yourself. I personally waited 3 weeks, and I was a heavy smoker. But I also did the JELLO technique, so I'm not really positive how I passed. (Whether it was the 3 weeks or the JELLO technique.)
I'm sure that doesn't help, but I don't really think there is anyone that can answer that question with 100% accuracy.