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. 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.