There's an industry standard for evaluating battery life for digital cameras, so many shots, so many with flash, so many viewings etc. I've never come across anything similar for computers and suspect, like you, that battery life is quoted on the basis of the lowest power consumption they can set it to and still do something.
That said, my Samsung NC10 claims a six-hour life and delivers it, but my Acer laptop claims 3 hours and delivers two.
I've never owned an Asus, just Asus components, but based on the latter, they should be OK.