Typical homeowners will put the detectors up, see them hanging there on the wall, and feel safe. They don't give them a second thought - except for those annoying times the low battery chirp goes off in the middle of the night.
Test Your Smoke Detector
Test your smoke detectors monthly and if they're battery-operated, you need to change those batteries at least once a year. Many people change their smoke detector batteries on New Year's Day or some other holiday to help them remember. Carbon monoxide detector batteries need to be replaced every six months (a great time to do this is during daylight saving time when you're turning your clocks back or forward). When replacing the batteries, vacuum the inside of the detectors to remove dirt, dust, and debris, which can interfere with the working components.
Detector Life Span
Most smoke alarms have a life span between eight and 10 years, and again, replace the batteries every year. A smoke detector with a titanium battery or a hard-wired smoke detector can last 10 years, at which time you would just replace the whole unit. The recommendation is to replace them every five years because their ability to detect carbon monoxide is questionable after that point.