It depends on everything inside of it. More ram, more CPU power, real gfx card, faster hard drive, larger screen all mean more power consumption. They are all factors.
I have a Core 2 Duo 1.8Ghz, 2GB Ram, 7400 Go Graphics card, 160GB 5400RPM drive, and a 15.4 inch screen. I have a 12 cell battery pack (which is larger than most) and get almost 4 hours on normal use. Also if you get Vista, or even XP, there are some decent power management options to get the most out of your computer.
And warlock, that would be true if the person has a Ni-MH battery that prefers a full discharge. Lithium ion batteries do not have a problem with the battery memory effect, and you will ruin the life of them if you constantly let the lithium ion batteries run all the way down to nothing. They prefer a partial discharge.