it all depends on the quality of the original wood. some is surface
treated, on others the preserative goes all thru the wood. preseratives
varied a lot too.

20+ years for good quality materials

The other issue is that the feds are considering declaring treated wood
as a health hazard that must be mitigated or removed. if that happens
it will be like asbestos.

your deck may have to go. at least before selling home

even rain water coming off deck is a cancer suspect polluting the dirt
under decks, which may have to be removed by a licensed contractor to a
hazardous waste landfill.

its another asbestos all over again