I hear the battery life will be very good. Of course your milage will vary depending on how you use your phone and how the strong the network signal is where you are.
One battery saving tip that helps me a lot is putting it on airplane mode when I'm in places with zero reception, saves a ton of battery life.
Also, you should drain it all the way then charge it up at least once a month. I think there's an article somewhere on this website about that...
I've got three accounts on push email, along with my corporate exchange pushing my calendar and reminders, and battery life is a non-issue on my 4. I'm guessing it will be pretty similar if not better on the 4S. Like others have said, if you're coming from Android, it's a non-issue. Battery life (or lack thereof) is the reason I left Android and waited until the iPhone 4S came out for Sprint.
Now if you were coming from Blackberry, that would be a different story....but it is still respectable. I easily can get through a day and a half without worrying about plugging in on my 4.