Lifespan of a wheel

Does anyone know what the average life span of a wheelset on a D&RGW narrow gauge freight car was? In looking at the cars that I have worked on (all of which remained inservice until the end of operations in 1968) I see wheelsets cast in 1918 that are well worn but still serviceable and wheelsets cast in the 1930's and 1940's that look brand new. I also don't recall seeing any wheel sets with casting dates past 1951 or so. Did wheels last long enough that the D&RGW did not need to buy any new ones past 1951, or were they scrapping so many cars by the early 1950's that they were able to keep cars going using wheel sets from scrapped cars?
Jason Midyette