If your changing elements.... Are you/ should you be changing relays? Then how long before wiring harness?
Relays have a different life span than elements, and a different life span depending on which brand of kiln they're on. If they run hot they don't last as long. I'd just let them run until they fry, but if you've got the cash it wouldn't necessarily be a bad thing to replace them every 2-3 element changes to prevent a misfire.
The wiring harness should be changed if the wire insulation is starting to discolor or becomes dry and crunchy when you bend it.