Depends on what I'm smoking, and how many times I've smoked that day. If I keep having smokes to keep it going, the last smoke will carry me much further than the first one will.

Assuming decent stuff, and a joint to myself... I would say about 1 hour, awsome high, 1 hour chilled out but still feeling a buzz, final hour feeling different, loose all physical high, and just kinda having strange thoughts still. After about 3 hours though, not really high at all. I usually try and smoke again during the second hour.