I use mine between 4 and 16 hours a day, (depends whether at home or not), and after a month the Keyboard and Mouse Bluetooth tab in System Preferences show up as both full bars, (keybaord used as much).
My guess will be about 6 months on current usage. I do intend to replace the Lithiums with Duracell Pro Cells though. If you want a long life, better off using good long life (cheap) batteries.