It really depends on what test they use, and what they are trying to catch. I live in a very pot-friendly community with MANY medical marijuana users (Including myself), and I and several acquaintances of mine have passed piss tests the same day as smoking by virtue of the test givers not looking for pot but instead for harder drugs (Meth and Heroin are huge problems in my city). Granted this is probably the exception to the rule, but it is worth noting depending where you live. On another note, I have also had non-drug using friends fail piss tests due to over counter painkillers, anti-depressants that were prescribed to them by their doctor, and even poppy seed bagels (Not making it up, it genuinely happened to a friend of mine, I didn't believe him until he had me watch the myth busters episode about it), so even if you don't have weed in your system you might still pull a false positive. Again, all these were for pre-employment, I have no experience with probation tests of any kind, nor hair sample tests.