Found this online

4) Battery life: I suspect that battery life will be similar or somewhat better than the current generation of Powerbooks. When I unplugged the MacBook Pro, it had a full charge and eventually listed 8:20 (!) as the estimated time remaining (Energy saver setting = Normal). However, I loaded up iPhoto and started paging through images like mad in the full-screen mode to tax the CPU and it dropped down to 2:30. This, of course, is thoroughly unscientific, but it leads me to believe that normal battery life on these things will be in the 3.5-4.5 hour rage.