detection times for chronic users typically range from one to three weeks but can extend much longer. Source

With EPIK, the medical test happens on the second or third day of orientation. Applicants run through the various stations of the medical exam one after the other. For the urine test, you'll stand in a line and receive a cup and you'll proceed into the washroom to fill it. People are allowed into the washroom 2-3 at a time, and there are no monitors present, so if you really can't cut usage one month before departure then you could always befriend the clean-cut guy next to you in line or have a friend give you a clean sample in a bottle and make the switch at the urinal.

Another option is a detox kit. In Vancouver I stopped into the Cannabis Culture shop and Mark Emery himself advocated the Rescue Detox kit. I cut usage 5-6 weeks before arrival so I didn't wind up using it, but when I asked about its reliability he said that they had never received negative feedback about it (if they had they wouldn't sell it).

Still, your safest bet is abstinence. It's a long and expensive way to come to chance your contract - if you fail the test, there's no redo and your visa is cancelled on the spot, so you're on the next flight home all at your cost. There's tons of information available through NORML.