As is so often the case - it depends. I have installed 6 of the thermostats, which have different replacement intervals depending on how often they have to regulate. Just under 1 year should be a good average.
You get a battery low warning on the app. But beware, the time until the thermostat switches to the safety position (open to prevent frost damage) is extremely short. I have also come home to find one room completely overheated. Therefore, check the battery status before a longer absence; there is also a local display on the thermostat.