This is one of those things where people will say YMMV, but IMHO no more than 5-7 years. That goes for hoses too. Yes the useful life is 5 maybe 10 in some cases, but any more is certainly pushing it. Why put in a spare that's nearing the end of its useful life? The items do wear in use, of course, but hoses and most impellers are made of rubber that dries and hardens over time and are affected by things like ozone in the air whether in use or not. My boat's timing belt was replaced last year and the mechanic told me to discard any rubber spares older than 5 years. OTOH -- I have rubber hoses still in use that are 11 years old and seem fine... guess I should be thinking of scheduled replacement.