I have 6 in the house. Batteries get changed when I change the clocks. Then they get changed out about every 10 years.
And yes, about 18 years ago, one did saved Mrs. Scout's life. Seems she had come home work (Nurse - worked night shift), started a fire in the fireplace to take the chill out of the air and went to sleep on the couch, at some point the damper closed and filed the house with smoke. The incessant noise of the Smoke Detector woke her up and got her out of the house.
Other then repainting the family room, no other damage. Although our clothes and the house smelled like wood smoke for about 6 months. :rolleyes: