Since this got bumped up, I'll chime in with my battery report with my estimated battery life given the data Windows has collected over 1.2 years of battery use. If you go to command prompt, you can type in "powercfg /batteryreport" to generate a file containing lots of stats about your battery. According to mine, given my usage of Internet Explorer, watching movies, and playing games on battery, I have an average full battery charge worth 6h45m which means that some days it's more and other days it's less than that.
Current estimate of battery life based on all observed drains since OS install
Since OS install 3:04:27
mind you, I have the power cover on 80% of the time. When I have concerts/weddings I definitely have the Power Cover. I also make sure to turn it completely off. My Surface continually wakes up from sleep.