It all depends on the environment and the treatment. These old poles that they called Black Diamonds were hard as a rock. I climbed one with a date nail in it from 1928. The date nail was put in to signify what year the pole was set. Let me go dig it up and show you hang on....

I climbed that when I was a 2nd step apprentice so that would have been 2006-2007. They can last a Loooooong time. Lol