In general the Macbook Air tend to have a battery life that averages over 10 hours. But there are a few PC systems that have really pushed the boundary up to 15 if not 20 hours with added battery packs.
As a general rule you will get more battery life from a smaller LED screen and a system that uses a Sold State Drive (SSD) rather than a spinning disk. However these features tend drive the cost up. Like a MacBook that starts at close to $1000, you can expect to pay close to as much for the equivalent in a Windows laptop.
Most of the newer Ultrabooks have long battery life, but they do tend to cost at least $1000. The longest battery life on a laptop that I've seen recently is from the Lenovo ThinkPad x230 (20+ hrs)
I personally use a Samsung Chromebook, and the battery there can last as long as 8 hrs, plus it's super light-weight (not even 2.5 lbs [1 kg])