Alcan, anyone that ventures a guess, is JUST guessing. There are far to many variables. As for Hvyfuel's link, again, there are far to many variables to calculate with any true degree of accuracy. Becides the fact that the link to the calculater isn't working.
Get a home test. They are available for $2-$3 on the web, http://www.meditests.com/drugsofabuse1.html, or about $15 at drug stores, or email me with a mailing address and I'll send you some free. http://boards.cannabis.com/showthread.php?t=1238
Home test, needless to say, are far more accurate than any guess I could give you, but given the info you supplied, I'd be surprised if you take more than 2 weeks to fall below the 50ng/ml testing cutoff level.
Hope that helps. N2