A Marijuana drug test can detect usage going back at least 30 days. Marijuana is a drug that remains longest in the body because the THC, the psychoactive substance contained in cannabis is fat soluble and has the distinction of bonding to  the fat and dissolveing very slowly. In view of the very slow elimination of THC, the periodic consumption of Marijuana a week apart leads to accumulation of the substance in the system and will cause a failure of a Marijuana drug test. In the long term consequences for the health of this storage are still poorly understood.
An estimated half-life of THC is about four days, which means that after 96 hours, your body has eliminated half of the amount consumed and even then it takes four days to eliminate half of the half and so on.

As part of a Marijuana drug test, the duration of THC detection depend on the frequency of consumption, as well as health status and metabolism of individual consumers.  In an occasional user, the metabolite of cannabis can be detected up to two weeks. In the chronic user (at least once a day) the duration of Marijuana drug testing can go 30 days or more.

The positive Marijuana drug test does not mean that the psychotropic substance is still present in the blood, and still active on the central nervous system. After Marijuana has been smoked or ingested, its main psychoactive component, the tetrahydrocannibinol, is extensively metabolized before being eliminated.

The Tetrahydrocannabinol (THC for short) is considered the main active ingredient of Marijuana, although other compounds present in Marijuana contribute to its physiological activity. The Tetrahydrocannabinol is rapidly absorbed by inhalation and through the gastrointestinal tract.  THC is present in very small quantities in the urine during the first hours (6-18 hours) after consumption of drugs, however, the THCA is then present in the urine in detectable by Marijuana drug tests for several days (10 days for a casual consumer and more than 30 days for a regular user).