This is coming from someone who used to smoke quality weed very heavy at one time, and has taken at least 50 urine tests, and a few saliva tests. However, my personal experiences should not be taken as a definite answer.
I have never failed a drug test in my entire life, and outside of drinking a gallon of an herbal tea the night before a drug test because it caught me off guard since I’d only smoked three days prior to it. I’ve smoked weed as much as three times a day for an entire month, and have passed urine tests without taking any special supplements or precautions outside of no giving myself a week to refrain from smoking, drinking water and exercising.
Unfortunately I know a little too much about this topic, but my advice will likely save your butt so listen to it. Urine and hair tests don’t detect the actual amount of THC in your body, but instead test for chemical changes via THC metabolites that have nothing to do with impairment. Blood tests on the other hand test for the actual amount of THC in your blood, so are more suited for determining impairment over general usage. Water doesn’t speed up removing THC metabolites from your system, but it helps your body to flush these out better, and it helps your detoxifying process work better since the body’s vital functions work much better when properly hydrated. Exercise burns more fat, and speeds up your metabolism, so more THC metabolites from the fat will enter your bloodstream, and reach your urine, though more metabolites will exit your body during a bowel movement.
Personally, based on what I know about this, my own experiences and advice given to me, and the experiences of others I’ve known, I’d say you’re 99% home free if you follow my advice. You had only smoked one time, so your situation is even better than others I’ve known, and myself in the past. Just do some basic exercise like walking, running or some type of physical activity in order to speed your metabolism up. You don’t have to run 5 miles or wear a rubber suit here, just do basic active things for at least an hour or two a day, and drink at least ½ gallon water each day, you don’t have to saturate yourself with massive amounts here. For the next five days you simple want to help your body do what it naturally does to function properly. You’ll want to slow down your physical activity about 2 days before the urine test but still drink water.
I’d purchase a cheap home drug testing strip from a drug or convenience store after about five days, and test your first urine of the day, preferably after you wake up since this is the time when the most concentrated amount of THC metabolites would show up. You know you’re home-free when you see the line show up on your test strip. I’d also recommend drinking plenty about a 20 ounce bottle of water, and urinate at least once before actually taking the test. You simply do not want to use your first urination of the day as your specimen as a simple precaution, but I don’t think even this will matter in your particular case. Don’t drink too much water though, or they’ll give you a positive test result anyways.
Personally I don’t even think you need to follow any of my advise above, except for the refraining from usage part, but those extra precautions don’t hurt. There are no guarantees when it comes to any type of drug testing, and increased weight, overall size, body characteristics and metabolism rates can make passing drug tests more difficult for some, while easy for others. This is why you should test yourself at least twice before taking your actual urine test.