I'd bet that most SP cleats are replaced when the plate and screws securing the retaining clip are ground down through walking as opposed to being worn out through use (un/clipping and pedaling). If you're not walking around on the metal plate/screws (by using cafe covers, keep on covers, SP's new walkable cleats, etc) they'll last a long, long time.

Also, I think that the maintenance of SP is often overstated and mostly by people that don't use them. A squeeze of grease in the pedals once in a while and a puff of dry lube on the cleats is easily and quickly done.