To maximize battery life the design uses the PIC's sleep and XLP (eXtreme Low Power) features:

• All unused peripherals are disabled.
• The MCU is put to sleep between sample & display updates (4 second interval).
• An inactivity timeout (e.g. no button presses) puts the unit into 'deep sleep' that powers down the display, sensor, and charge pump.

In deep sleep the bias current is ~ 32uA which gives the unit a theoretical shelf life of about 260 days with a typical 200mAh battery. When on, but with no user navigation, it is ~ 910uA with sample peaks of 6mA for 10mS. Ignoring the peaks you'd get about 9 days of use with the off timer disabled. So, actual battery life depends heavily on the amount of use and timeout setting.