I get about 3.5 to 4 hours. The power save features are predetermined and kick in around 20% battery left. It slows the processor down a bit as well as killing unnecessary things that might be running. Also brightness is adjusted too. I usually have my brightness at 35 but when on auto, I notice once the battery is below 20, the screen is dim unless there is a large change in brightness around me...
HTC Droid DNA -Rooted Stock
Try Out DropBox, get your files when you need them, its free http://db.tt/LUr0eAv