i cannot use a clean persons sample because it is random, they will test me on the spot.
There are countless posts in this forum of people who have successfully subbed for a random U/A.
From your point of view, how many days would you "think" it would take for the thc to get out of my system considering my size, etc. i am 145 lbs 5' 5". 30 yrs old. An educated guess if you can please? :
Nobody can answer that for you. Each persons body is different and there are too many variables to take into consideration to even arrive at an educated guess. Since youve expressed the importance of passing this test, why would you want to rely on "educated guesses" which are almost always off the mark?

Only periodic testing with a THC test kit (using the first urination of the day for each time) can give you that answer to your question.
also if I drink plenty of water and cranberry juice and bike riding everyday. Also do you know of any foods that help burn fat? Thank you
Cranberry juice does nothing to rid the body of THC. Drinking water is only helpful on the day of the U/A. Drinking water days or weeks before the U/A is of no help. You must understand that THC is not water-soluble. THC is fat-soluble. Therefore you cannot rid THC from the fat cells by drinking copius amounts of water. It does not work that way (more on that in a moment).

THC is stored in the fat cells. When fat cells burn, the THC metabolites are released back into the bloodstream. The kidneys filter the THC metabolites from the bloodstream and are deposited into the bladder for the eventual exit (this is the main reason why THC carries a longer detection period than other types of drugs). The rate in which THC metabolites are filtered from the bloodstream by the kidneys is unaffected by copius fluid consumption.

The purpose of drinking fluids is to dilute the THC metabolite concentration that is present in the bladder. You are not ridding the body of THC metabolites, but rather youre "thinning out" the THCA concentration present in the bladder. The idea is to thin out that concentration just enough to where the urine sample can register below the cutoff thresholds of the drug screen. This dilution phenomena, however, is only temporary - once the fluids ingested had all been urinated out, the bladder is replenished with fresh THCA-laden urine by the kidneys, and thus the donor would resume testing positive for THC.

As stated previously, only time and exercise can permanently rid the body of THC. However, if your scheduled U/A happens to be 3 days (or less) away, then fat-burning activities can work against you due to the reasons stated in the above paragraph. Therefore, for those 3 days, youd want to curtail any fat-burning activities and be as dormant as possible until after the U/A. That way you wont be releasing a bunch of THCA into the bloodstream, which would all end up in the urine to be detected.