The time it takes for THC from marijuana to leave your system depends on how often you use it. If you only use it once it should be gone after 8 days. If you use it fairly regularly then it can be in your system for up to 15 days. If you use it every day then it can last in your system for up to 30 days. If you use it heavily every day(more than twice a day) then it can take up to 44-77 days for it to leave your system.
The easiest way to think about it is that if you consume marijuana regularly than the THC can remain in your urine (the most common way of testing for THC) for up to one month. Urine testing is pretty accurate and also very inexpensive for testing whether you have THC in your system or not.