An article in about About.com regarding how marijuana metabolizes in our system, points out some of the limitations of drug tests and explains how marijuana’s active ingredient, THC, is processed in our system.

It also explains what stays in the system — the metabolites in marijuana which leaves our body through our excretory system — and gets people fired for testing positive in urine drug tests. These metabolites have a “half-life” of about two weeks, but heavy smokers have had a problem up to 90 days, although there’s no authenticity to this claim.

From what I hear, it’s about 50/50 on completely washing out your system with a masking agent, unless you stop smoking at least 48 hours beforehand — any closer than that and you’re taking a pretty good risk that something positive will show up in the results.