Well... the duration of a high depends on several factors... the amount of weed you smoke, the amount of time you took to smoke the weed (like, bongs burns weed faster than joints, etc), the strain (indica/sativa), your tolerance, the amount of food you ate before and after smoking, etc... (there are several others but i cant remember... :stoned

My own highs lasts about 1 hour at most... the first 20 minutes are pretty intense, and after it the high decreases steadily... after 1 hour im only buzzed and after it im almost sober again.