how long is a piece of string? Depends on the load placed on the machine.
video editing with loads of HDD accessing alongside CPU/GPU utilisation while listening to music from a CD in the DVD drive will use a huge amount more power than using the machine for some fairly light word processing.
The battery life will be from approx. 1 hour when going maxed out to about 3 hours tops when using it for anything more than a screen saver. You could expect to perhaps push past the 3 hours if you had all the power saving features activated and are not too hard on it, but its tough to say exactly. You will figure it out within a few days of having it.
Battery life will also vary with the OS you are using. Linux distros generally are more power hungry than Windows. That is rarely an issue as the vast majority of users use Microsoft but worth bearing in mind if you are going to change or dual-boot.