Ok, units - How often are you supposed to check it? How long can pass whilst its being fit for purpose? does it or should it be checked as in terms of rotable type calibration?
The justification for it is sound , makes sense and as you say price wise... well , what price could be put on that of a life of family.
I check ours every month - by that, I check there is still a green 'OK' light on it. Basically, the main cause of failure would be battery deterioration and that's what the inbuilt self-test is looking at. The units themselves come with 8-10 yr warranties (typically) and one is advised to replace them in this time. The batteries are not warrantied for that time but are self-tested - typically 5-7 year life.
Edit: you can buy unused, batteries replaced, units taken out of service but I decided the price difference didn't justify it.