Depends how you use it, they rate a max of 14.5 hours but that would be with low cpu usage and low screen brightness.
My Surface Pro 3 lasts anywhere from 2 to 7 hours of use. 2 hours playing a game, 7 hours just reading without the keyboard attached. Sort of like a phone, I usually get 24 plus hours from a charge, or I can kill it in 5 if I'm gaming on it.