According to NORML, the average detox time for chronic users(on the 50 ng/ml test) is no more than ten days, and 2-3 days for occasional users. On the lower cutoff, it shouldn't be more than 20 days for a chronic user. This is according to the NORML article "Marijuana Detection Times Shorter Than Previously Assumed"(Feb 2006).

I'm not entirely sure about it, but I trust NORML a hell of a lot more than the DEA.