When I bought units for myself and my Mom and my sister, I was informed that the units are effective for about five years. They have battery indicators on their face.
I was told to take a sharpie and write the install date on the unit and then replace the unit five years from that date.
Thanx. I didn’t do that, but will be certain to do it the next time I buy them.
The beeps were loud enough to burst an eardrum when I changed the batteries, so I think they’re ok for now.
The info says to check them every week.