I assume protectors eventually wear out (assuming they're the MOV-type), so, how often should they be replaced?

I've replaced carbons in those that are probably 40 years or more old. I've never heard of one wearing out.

I should have been more clear, I really meant the secondary protectors from ITW and Panamax. I assumed that since power line protectors wear out, so would phone line types.

With solid state it depends on how much they have been "used". They should be OK unless they take some heavy hits. I like to use 5 pin protectors with fuses so it's going to be obvious when that happens. Other types may not show any signs of damage unless they fail with a short. So to answer your question I don't know either. I would think if I were in a lightning prone area I would want use what I use.

-Hal