Too many variables to try to answer. Like trying to figure out how many miles you can get on 10 gallons of gas. Need to know whether you are driving carefully, or hot-rodding! In your case, what OS, how much memory, what size hard drive, what programs are running in the background, ambiant temperature (if the room is hot, the computer will run hotter and use the cooling fans more). To be safe, carry a full spare battery, and back up frequently (of course, backing up more often will use the hard drive more, using more power...)!