Depending on the power settings used and what you do with it, i.e. gaming, internet surfing, etc..., you'd probably get 2-3 hours on a mobile power plan. A high performance plan would probably put you in the 1-2 hour range (generally speaking). Of course, that is estimated on the 6 cell battery. If you went with a 12 cell battery, expect those times to double.
I would say that is quite optimistic, but if that's the case then sweet!
I would say 2 hours max to be honest with you...