While it's hard to find solid science behind the recommendations, home smoke alarms are suggested to be replaces at least every 10 years. That recommendation assumes the detector is ionization or photocell based.
CO dectectors are different.
That said - I still recall vividly hearing the news of a family of four dying in an Aspen luxury home because of CO poisoning.
When we got the RV, I not only replaced the batteries in the existing alarm, I spent $15 for an extra CO alarm. Simple to mount. Two screws.
While the old adage might be "Man with two watches never knows what time it is" I prefer the "Man with lots of alarms and detectors sleeps better." :)