The 2 answers above (and mine) are proof that no one answer for one person, can directly tell you how to handle your device.
I leave mine in the lower 25% of the slider, most of the time. When I am outdoors in the sun, I do push it up quite a bit, though rarely all the way. Indoors, normally, about 25% is plenty bright enough, and of course in the dark, I take it all the way down, and sometimes use a screen dimming app to take it lower.
The only time I push it higher indoors is if I am watching a video, or showing off a photo to someone, etc.. and even then rarely over 50% is necessary. I don't use auto, because I hate seeing the screen change brightness by its self while I am using it. For me its also not a battery thing, its just where I am comfortable.