Different factors affect how long THC stays in a person’s system. These factors can vary greatly from person to person, and include the person’s metabolism, their body type (weight, height, etc.), how much marijuana they smoked, how often they smoke, how long they smoked, and how much THC was in the marijuana.
For occasional users of marijuana it has been estimated that many blood and urine tests will detect marijuana for about 10 days. For regular users, that number may increase to 45 days, and constant users may have marijuana detected in their blood or urine for as much as 90 days.
Testing can also detect the by-products of THC, which are often referred to as metabolites. The length of time metabolites can be detected also varies greatly depending in part on how often it is used. Metabolites have been detected for an average of 13 days for casual users and for more than 90 days in some heavy users.
Because marijuana is stored in fat cells in your organs, it is potentially detectable for a very long time—even years. However, it is appears unlikely that it would be released or be detected by current blood or urine screens for periods exceeding those identified above. It is of course possible that more advanced detection systems will be developed.