Battery life on auto brightness

Your battery depends on many things, it isn't just a phone with 1 function so you can't say exactly how much time it stays alive with auto brightness

Althought i bet it saves quite a bit and it's really good to use at night, it doesn't hurt my eyes because screen isn't as bright as on low brightness
 
I leave all my settings (Brightness=Auto, WiFi, NFC, Bluetooth, etc) on by default, and often get 3 to 5 days of usage. Note: I don't use cell data (Only WiFi) and I turn my phone off when sleeping.