To clarify my last post, she has an 11" Air and this is in comparison to my old iBook G4 where I played Unreal Tournament 2004 for three hours and 45 minutes before the battery died, which is exactly the amount of time I waited in the ER to get an injury checked out. lol. Mind you, UT2004 is not a CPU taxing game, it ran great on the G4.
On the other hand, my wife used the Air on and off over eight hours and by the end of it still had 40% battery life remaining -- listened to a few hour long Podcasts (letting the screen shut off, it uses very little battery), browsed the Web tethered via Bluetooth to her iPhone, etc. Her battery settings are screen off after one minute, sleep after two. With virtually instant wake up like an iPad, you wouldn't know it went to sleep in the first place.