I recently had to take a pre-employment drug test... I am a pretty avid marijuana smoker... so I was freaking out because I only had 10 days to clear my system. Low and behold... I took 2 home tests (1 3 days after my last smoke sesh and another a week later) and passed both...
I gathered a ton of info about drug testing including a family friend who happens to be one of the top oncologists in the country (formerly worked in a drug testing lab) and he laughed when I said I was freaking out... 48 hours for someone who is of normal weight and has a normal metabolism. All "flushes" and other such 50-60 dollar drug cleansing kits are only trying to make money off of those desperate enough to spend it to try and pass.
Drink water...cranberry juice...eat healthy..whatever. I passed my test with flying colors. It's very rare that if you refrain from doing drugs for 48 hours you'll fail a simple urine test.