I don't really know how long it will run in total, but I tried mics > BB > recorder at 16/44.1 with an 8 GB microSD, plus the built-in memory, on 2700 mAh batteries. It went until the memory was full…Almost 19 hours (18h46m) and I still had a couple of bars. That was with the backlight (and LEDs) on continuously.
Personaly, I have found somewhere between 3 and 4 to be a good place with respect to levels...YMMV and all...