For those who are averaging 6 hrs battery life, even after several full battery discharges and recharges, you should return your phone (or go through the warranty). That's just not average, normal, or acceptable with this phone, you might have a lemon device.
What do you consider heavy use, because if I'm listening to music and playing games and such which I would consider heavy use, I may get 5 hours on my Lumia 920. If I occasionally check facebook, read WP central and not use my phone much, I may get 8 to 10 hours use.