mine can last as long as 2-3 hours sometimes longer depending on how good the pot is and how much I smoked.
If it's the first smoke of the day then 3 hours or so, if I have been smoking then it's like an hour. I think difference is just how much I notice it. If I'm already high then I don't notice the difference that much.
Depends on the strain, really. I smoked half a spliff of one kind of grass once and I was only high for thirty minutes, while I smoked a bowl of Orange Haze another time and I was high for three hours.
Depends on strain and tolerance. Schwaggy indica mexican stuff can have me reasonably high for like 8 hours, with no tolerance. Smoking more often, 3 is when I'll be pretty much down, and want to smoke more. 1 is great, too, though.