How long will weed stay in my system after one hit?

I'm a male, 5"10, weigh 167, I work out at least 30 minutes to 1&1/2 hours a day, I have a fairly fast metabolism because as much as I try, I can not gain any weight. I took one snapper hit of decent bubba kush & held it in on January 12 at 1:00 AM, I have a possible urine test on Monday January 17. Before that I hadn't smoke for 40 days & I only took 3 puffs of below average grade stuff & before that it had been 30 days. How long do you this one snapper puff will show at a 50 ng/ml cutoff level urine test considering I drink a **** load of fluids, help me out folks! thanks...