Question about TFA Casa

I bought my TFA Casa in 2016 and set it up in my holiday home. This is in operation for maybe 2 months a year. When no one is there, we take the batteries out of the devices. I don't remember us having to change the batteries in those 4 years or so. This would mean that the batteries last for a few months for sure. This is also the case with another similar device at our house.