My F250 had 30K miles on the 19.5's when I sold it and they still had healthy tread. OEM 17" Continentals used to last only 35K miles to the wear bars, but I could buy those cheap as dealer take offs. I sold my 18" Michelins with about 30K miles on them and they were probably past their halfway point when I switched over to the 19.5's.
My 5500 has over 45K miles on the 19.5's and they will age out before wearing out. Both trucks average 10k miles a year carrying the camper with few miles running empty.