Battery life expectancy
The D-535Z uses two AA batteries, and Olympus includes alkalines in the box. Unfortunately those won't last long and will end up in the trash. So I recommend buying a four pack of NiMH rechargeables (plus a charger), which gives you a set for the camera plus a spare. Unfortunately Olympus doesn't publish any battery life statistics for the D-535, so I can't give you any exact numbers.
It depends a lot on how much you use the flash and LCD screen. Don't leave the screen on all the time, use it sparingly and you should find an increase in shot numbers.