Depends on the pin. Newer pins have stronger flipper coils and lots of steel parts that can really beat a ball up. Older EM pins can be very gentle on balls and rust in the first thing I see damaging them.
Look at the balls and change as needed when they show pitting, scratches or damage. Some of my pins see a lot more play so the balls get beat up quicker. Others, sadly have rust set in before mechanical damage.