Oxygen sensors on newer vehicles; how often should a change should they be changed?

I have 1987 Dodge 1/2 ton pick-up and a 1991 Ramcharger. They both run really well. I have them maintained as best as I can. There is the 318ci engine in both vehicles. One has a carburator and the other has a throttle body injection system.|||on really new vehicles most don%26#039;t recommend until after 100,000 miles or until they fail. on older vehicles probably 30,000 miles because their emissions systems aren%26#039;t as advanced they foul up sooner.|||as soon as they go bad! when u failed smog test it could be one of those factors!|||They only need changing when they set a %26#039;diagnostic%26#039;! You%26#039;ll know this by the check engine light.|||ALL OF THE ABOVE, THE CK ENG LIGHT WILL COME ON WHEN IT DOES SO YOU CAN GO TO SOME PARTS HOUSES, NOT ALL OF THEM, %26amp; THEY DO A COMPLIMENTORY DIAGNOSTICS FOR YOU %26amp; LET YOU KNOW WHAT THEY FOUND