When they chirp. At the price of batteries these days? No way I'm replacing them 2x a year. I have 6 of the things in my house.
I've got neighbors who go YEARS (no joke, had to listen to it) without changing batteries AND they listen to the thing chirp every 45 seconds.
I've concluded using Duracell or Energizer batteries changing them once/year is fine and perhaps overkill. I have 5 and one of those has a light so it takes 2 batteries. I've yet had any of them chirp and all test ok.