This information is presented in the user guide/datasheet for the device, which can be found on the downloads tab of the product listing.
However if you are using a mobile phone or tablet to view our website it may not be visible.
To this end I have copied in a link to the electronic version of the instructions as well as a brief summary of the requested information below for your convenience.
(bottom of 1st page)
The batteries in the Wireless Dummy Siren should be replaced every 18 months
Date published: 2016-09-09