I work in an industry where piss testing is required before employment AND post incident. To this date I have yet to piss positive, and it's been over 8 years. There are TONS of myths that surround this subject and you've really got to approach this using your own common sense with the REAL facts in front of you.
If you are someone with a high BMI such as 28, you will piss positive for a longer period of time compared to someone who is say a 16.
Why is this?
As most people know, the THC is stored in your fat. More fat means more THC that will be stored.
Half the people say to exercise more and you will cleanse your body of the THC. This is true to an extent. Hear me out...
A couple days before your 'exam' DO NOT work out! You absolutely don't want THC to get released into your urine at this point in time.
Cranberry juice? No. This is just a myth. I wont even entertain this one bit.
Herbal THC cleanse kits? Yes, they work BUT only because of 1 reason...the massive amounts of water you are required to drink. All that other junk you consume is merely a placebo.
I was a fairly heavy smoker back in the day. I had 7 days to clean up for a piss test. I passed with flying colors...3 times. I have 2 trains of thoughts on how to pass, both have worked for me. Here they are:
1) For 1 week you need to drink a TON of water. You will want to take a piss 2-3 times per hour. It sucks, it gets old in a hurry, but you MUST do this.
2) NO ALCOHOL. They will work towards dehydrating you. You want to be ridiculously hydrated for 1 week.
3) NO CAFFEINE. See #2
4 a) When it comes to the test, you want just the slightest bit of yellow in your urine. Most tests require more then just water in your piss. If it's inconclusive, you just have to piss again. Your sample will have a bit of THC in it, but not enough to come out positive due to how diluted it will be.
b) Instead of running a highly diluted sample, piss VERY yellow...like syrup. Why you say? Your body will have no THC flush in your bladder. It will have all gone out in your last few pee's. You will struggle to get to their line on the cup and even be just below it is fine. They will tell you it should be enough, and it is. This sample will contain whatever the hell your body/bladder produces (I'm no doctor) with a small amount of water.
Little long-winded, but I hope it helps.