Davis, claims it will last 3 years, but really I would change it every year… pick a season…
I usually change mine twice a year… but then I have a real good deal on CR123’s from the Battery Station and I buy in bulk. I have all those 6 extra sensors running and of those, they do use the battery more.
Niko is right… do it annually. One of the things anyone needs to plan ahead is where are you putting the ISS and is it easy to get to to do the replacing and hence… pick the season so if you have mounted it on a roof, is the weather such you can go up and come down gracefully.