My new house has hard wired detectors, and I didn't realize they also had batteries.
A little less than 2 years after moving in, I found out the batteries needed to be changed when the alarms started sounding at 0300. Not the learning experience I really wanted.
Then about 9 years after moving in, the detectors started chirping . . . again, at around 0300. That's when I found out the "average" life span of these detectors was "about" 10 years. Seems mine were on the short side of the "average."
This was 11 years ago . . . I replaced all six detectors then. Pretty easy, since the new ones fit the old wiring and old sockets. No problems since.
(Hmmm . . . 11 years on 10 year average life span detectors. Tick . . . tick . . . tick . . . )