OP READ THIS
This really all depends on your body-type, metabolism, how much you smoke, and how often you smoke.
Let's just say if you're thin, and have a fast metabolism, then you already have a plus side. Marijuana, or THC rather, takes about a month in the AVERAGE person. This is not to say it won't take shorter/longer with most people. But this is all going by a urine test.
Most likely you will being taking a Quest Diagnostics test, and probably SAP 7-50/300 GC/MS, or something along those lines. (A lot of companies are beginning to use this because it is accurate, and affordable.) That basically means you will need to be clean. Drugs are measured in ng/ml, and detectable for cutoff is 50 ng/ml, max able to be read 300/ng/ml, that's pretty good, as these tests are known to go as low as 15ng/ml (the smaller the # the worse for you, as they are trying to see how much thc is in your blood per ml).
Ok, so enough about info. Now what do you need to do to pass? Well, you only have 2 days, or maybe 1 now as you read this, so it's going to be hard. But since you say you very rarely smoked (I hope you didn't take too many hits), don't worry you can do it. Up till tonight, I say don't eat ANYTHING, and excersize. Why? THC gets stored in your fat, and slowly releases it into your bloodstream, so you wanna get rid of that fat that you might have added on in 2 days. Drinking a lot of water periodically helps too, but you wanna save that till an hour before the drug test. Ok, if you've done that, what you need to do next is, by dinner time, and all through till the drug test, you wanna start eating junk food, and when I mean junk food, try and add on as much weight as you can by the test. Because you wanna get a nice clean layer of fat over the THC filled one. Since it's pretty late, you may just think about skipping the excersize altogether and just get a layer of fat. Also you can run over to the local CVS/Eckerds/Genovese, and get a home test kit. These work. If you pass these, you are probably good to go. However, they go for $20 I think. If you decide to go excersize first, take some caffeine pills if you can (these can be bought at CVS too), as it greatly increases your metabolism.
Drink plenty of water (but don't over do it, I'd say about 8 glasses), an hour or two before you get to the test lab.
That's about it, oh yeah, and pray.
Oh yeah, if it's a hair test, you may be screwed. Unless you wanna go CheapyD style and shave off everything. There really is no way of cheating that, as THC stays in your hair.
And 1 more thing. There's another route you could go...that's subbing. You could sub you're urine. There's plenty of online sites that sell synthetic urine, b ut as you're short on time, you can ask a friend of yours that you know is clean (make sure no medicine, pregnancy, and alcohol), and take theirs. Store it in a bottle, or container, and try and keep it heated at 98.6 degrees until test time. This is trickier, but it is the most fool-proof way.
Good luck!
*edit* There are studies that say aspirin works as well. Take these periodically to thin your blood.