There is no exact time for changing the batteries because it depends on the battery performance which differs from different brands. Also, the usage environment affects the life of batteries too. According to our test, the Duracell batteries can still work at the voltage of 2.4V.
So, you can watch the battery voltage changes and set an alert for low voltage, such as 2.5v. For WS1 device, the batteries are expected to work for 4－6 months under default settings. While, as for WS1 Pro device, the batteries can last for 8-12 months.
If possible, we recommend you plug the device to USB power with batteries installed, which is more reliable, because the batteries can be backup and you can also set alerts for USB DOWN. This also helps you to monitor the conditions of power off. You can have completely peace of mind then.