MOV replacement

How often should the MOV elements in a surge protector be replaced? I know they wear out after a few 'zap' cycles.
I have a bunch invested in my Panamax and would be willing to replace the MOVs in it to 'renew' it.

how long to those things last? I don't think there has been any lightning surges in years and I can't remember the last outage. The voltage at my house is always around 117+.