How much time does thc last in system?

Depends on what you: mean by 'system.' That is really not a medical word, and in many ways is meaningless. I suspect you are asking how long it shows up in a drug test, and that is a totally separate question. THC is very short-acting and lasts a couple hours in blood, which is why the high from single use lasts a couple-three hours in most people. The metabolite, however, lasts for days, but is not psychoactive.