In the anandtech reviews they set the screen brightness of the iPhone 5 at 200 nits in their battery life benchmark and managed to pull off 8 hours with it. So where on the brightness level slider would I get the equivalent of 200 nits?
It will vary from handset to handset, but generally is around 70% (probably with the slider close to the "&").
However the biggest factor in battery life seems to be how hard the cellular radio is working to maintain signal. I can get 9-10 hours on home wifi, but only 5 - 6 on cellular at home/work were the signal is poor.
The other usual suspects in poor battery life are apps/services that use GPS, bluetooth and push mail.