Too many variables to answer this one. There's the strain, the time of day, how long since the last smoke, the ambiance, the company, the activity right after the smoke.. and so on.

If I had to throw an average, the inital buzz lasts about 10 mins or so. Body high and droopy eyes last anywhere from 30 mins to an hour and full blwn sleepy feeling is gone in about 2-3 hours. These numbers can very greatly depending on the things listed above.