Your alarm will indicate end of life seven or ten years after initial power. It will 'chirp' every 30 seconds. The chirp will not stop until the unit is powered off. You'll know that it is an end-of-life warning and not a low battery warning because replacing the batteries will not stop the chirp. The chirp will stop only when the alarm is powered off.
Note. The alarm will not detect the presence of CO when in this mode. Replace the CO alarm with a new one immediately.