How long does marijuana last in the body?

According to the website of Columbia University's Health Services:

Marijuana itself does not "stay" in your system, but the main chemical in marijuana, THC, does. It fact, it may stay in your system forever because it is stored in fatty tissues of various organs.

Despite being stored in body fat, THC is onlydetectable for a certain amount of time. Many drug tests, including urine tests, look for 9-carboxy-THC which is the by-product of your body metabolizing the THC in your fat.

Marijuana Detection TimesFrequent users blood drug test (defined as someone who smokes several times per week)

THC: 4-8 hours after smoking

THC by-product: 2 or more weeks after smoking

Frequent users urine drug test

THC by-product: 2 weeks to a month after smoking (3 months for extremely heavy users)

Infrequent users blood drug test

THC: 3-4 hours after smoking

THC by-product: 2-3 days after smoking

Infrequent users urine drug test

THC by-product: up to 10 days after smoking