Do you live in a state that had medical cannabis? My employer, and the union both tried to force everyone to take a medical cards to HR directly if they had one. I did some reading, and only showed the person collecting my actual sample my legal card and prescription. The UA collector said I was correct that I only need to share records with the collector or Dr, not HR or management. I'm sure it's different for truck drivers, but just some personal experience.
If you just drink a gallon of water an hour before your test, your cretenine levels will show diluted. I've smoked the night before a test and took in of the drinks you get a head shops. They work, but file directions precisely, and you must know the hour of the test because they are only effective a short time.
Length of time to get clean depends greatly on body fat. Thc is stopped in fat cells which causes it to take much longer than other substances to flush. Skinny guys got it much easier.