It's been so long since the last time I smoked that I can't even answer that. (My current occupation has random drug tests with no warning and I also have a secret security clearance, so there's no way in hell I would risk it.) But... I DO remember the good ol' days. My high lasted all day because I rolled my own cigarettes with menthol tobacco mixed with my lovely Mary Jane... It's been 14 years since I last had the pleasure.
I don't smoke that much anymore but the last time I got toooo high I'd say maybe 2 hours or even less. It was enough to walk home (25 minutes) and watch most of a movie. Otherwise, I just smoke a small amount and it only lasts 45 then like other people said, I'm spacey for a bit.
No matter what i smoke, and no matter how high i get, it only lasts 45 min to an hour now tops. My tolerance is shit now, but i love it so.