He takes his phone off the charger at 0330 every morning and it lasts him until about 2200 at night, then he just plugs it in while he's sleeping and repeat.
I always kept mine at half because it seemed about right to me, but now that winter seems to be ending and I actually go outside I am trying out the auto setting.
I keep my 3/4 brightness. Turn it auto on when I go to bed. Plug it in rinse and repeat until Saturday I leave it on from Fridays charge and let it run all the way down. So, it lasts from friday 7:00 am until about 10 or 11 pm Saturday depending on use.
I'm always at 100% brightness, except when I put my N2 in its car mount in the morning in the dark. Then I turn the brightness all the way down. Anything brighter is too distracting in the car when it's dark.