The Compact Backlit Bluetooth Keyboard has a built-in 750 mAh battery. 

If you are using the backlight all the time at the highest brightness, it can drain the battery quite fast, but it does have a standby shut-off mode, that kicks in after 30min of non-usage. You can change the settings so it does not completely shut off.

Here is a chart showing estimated battery life:

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With no backlight on, the battery is about 60hrs.

If you would like to use the backlight and not worry about the battery, you can also use the keyboard in wired mode. While using one of the two cables included, you can use the key combination (fn+usb/eject) to turn off the Bluetooth and use it in wired-only mode. A green light will stay lit on the top right when in this mode.