With a 110mAh battery, expect around a week on the left half and a month on the right. This is an estimate and can vary. Leaving the keyboard on when your device is off, transporting it without turning it off, and connecting to multiple devices can drain the battery faster.
You can potentially upgrade to a slightly larger battery (401230), but it's a tight fit and might require a different case. The 301230 is the recommended size.