Depends what you are doing with it, what brightness you have it set to (screen lighting is just about the biggest power draw), which apps you are using or have running in the background etc etc etc.
I get about 8 hours out of mine for some light web browsing (with adblock and click to flash plugins installed in safari) or office use or video watching in VLC at 70% screen brightness.
That can of course vary wildly and if you were pegging CPU and GPU in a game I'd be surprised if you got more than 2-3 hours....