Most 15 inch laptops comes with 2.5 to 3 hours of battery life in general. Although the company promises 3.5 hours of battery life you should be very clear about the fact that your laptop battery holds good for only 2.5 hours to 3 hours of battery when fully charged. 14 inches do comes with bit more battery life but be sure to check for the reviews on this one from those who already own the model you are looking to purchase.
There are whole lot of other factors you want to consider when purchasing a laptop but these five is something which is really essential that you need to pay a little attention to.