Cast Iron should have a shelf life of 75 to 100 years. Unless its a very old property they should not have to be replaced before than.
https://www.houselogic.com/organize-...eir-lifespans/
PVC on the other hand has no life span as far as we know it. Right now they are classified as " Indefinitely" which could mean over a thousand years.
Who knows really how long PVC can last, for now we at least know it is a very long time. Not Geologicly long but for several centuries at the least.