depends on quality, quantity and little bio-chemistry:

are you fat or skinny? have you eaten/are you hungry? taken anything else? are you drunk? do you smoke regularly? do you have high/low tolerance? how much you smoke? how do you smoke it? what are you smoking?

its different for all of us but i think the average is about 15-20 mins peak and about 2 -3 hours coming down. what do you guys think?