[SAI_Peregrinus]

True, this does depend on the detector type. Photoelectric detectors last pretty much indefinitely, but ionization smoke detectors do age. And you ideally want both, since they detect different sorts of combustion. Also there are smoke detectors you can buy with a 10-year life lithium primary cell, so no need to change batteries. These are perhaps a bit wasteful (the cell isn't replaceable and the rest of the detector could be fine) but also don't require throwing away lots of 9V batteries.

[tlavoie]

Oh, true enough. Or at least, by the time the battery gets low, the whole thing should be replaced anyway. As you say, depending on detector.