|Well, I haven't gotten there personally, even after two sons through the diapers. If the snaps go, that would certainly merit replacement for me (I've never heard of people's snaps breaking, and I guess you could use something like a snappi if necessary). As the diapers age and wear they would begin to get thin in spots. They would still be useable, but might require extra absorbancy to function well for the average changing interval. There have been people on this board whose diapers have gone through seven or more children. ME certainly doesn't guarantee anything of the sort, but they do just keep on going. Longevity is going to be affected by treatment, obviously, so if the previous owner used bleach regularly, that would shorten the lifespan, and someone using a larger stash should see longer wear per diaper than someone with fewer diapers, since obviously they'd be getting used at different rates.
All that to say, I would guess you'd know if you really needed to replace your diapers because they'd be threadbare and not absorbing enough (I guess).