Most cement (not all of concrete, just the cement part) passes through a ball mill during manufacture. Mining operations that are followed by chemical separation usually have a ball mill step. That gives a sense of the scale of the ball milling industry.

Replacing the steel or iron balls is easy. Replacing the drum liner plates is also required, maybe every six months. The plates un-bolt.

More than you wanted to know about ball mills [1]

[1] https://www.flsmidth.com/en-gb/products/milling-and-grinding...