How big the battery is and what amount of drain there is.
You can have a 1,000 Watt laptop and get 20 hours of battery life if the battery was the right size. This is exaggerating the answer, but you can't measure battery life except to get your setup and ... test it.
Bob
i know that centrino means having an intel pentium m processor, intel network and wireless cards, and intel built on graphics. if i have all the components, except something like a geforce 128mb graphics card rather than the built on intel graphics, will there still be a decent battery life, or is it really fussy about having intel components??

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic