You should be notified by a push notification when Monimoto battery level falls below 20%. Do not wait until they are completely empty and replace when the first or second notification is received.
You can also check the red LED on the device. Remove batteries from the slot and wait for 2 minutes, then insert back. If it is blinking one long flash every second or does not blink at all - the battery is very low.
In general, Monimoto batteries last up to a year. If you received a lot of alarms your battery life might be 3-4 months shorter.