I averaged anywhere from 5 to over 8 hours of usage per charge on my 5S on iOS 7. The faster I used the battery, the longer it lasted. (Meaning, if I use it heavily all in one day, I'll get over 7 to 8 hours of usage, or if I use it lightly, spread out for 2 to 3 days, I'll get 5 or 6 hours of usage)
On iOS 8 I get around the same battery life. I haven't done any full drains down to 1 or 2% to take a screenshot in a while though. I'd imagine the iPhone 6 should easily do as well as this, or better,