When you smoke weed or Marijuana the effects of smoking the Marijuana will fade pretty quickly but the weed can be detected in your system for long after the effects have faded away.
If you plan to take a drug test then don't smoke any Marijuana until after the drug test has been performed.
Some THC metabolites have an elimination half-life of 20 hours. However, some are stored in body fat and have an elimination half-life of 10 to 13 days. It takes five to six half-lives for a substance to be almost entirely eliminated.
This is why you see advice that one-time use is probably not detectable after five to eight days. Most researchers agree that urine tests for marijuana can detect the drug in the body for up to 13 days.
However, there is anecdotal evidence that the length of time that marijuana remains in the body is affected by how often the person smokes, how much he smokes, and how long he has been smoking.
Regular smokers have reported positive drug test results after 45 days since last use, and heavy smokers have reported positive tests 90 days after quitting.