A urine drug test can detect marijuana as soon as 2 to 5 hours after use. However, the bigger question is how long will it still be detected with a THC urine drug test.The length of time marijuana is detected in urine depends on the amount used, frequency of use and the user's weight and body fat. urine drug test can detect marijuana as soon as 2 to 5 hours after use. However, the bigger question is how long will it still be detected with a THC urine drug test.