A very interesting question.... I did not find any maintenance info on cap/rotor in the service manual.

Looking at the Helms manual, it says "check the distributor cap for cracks, wear, and damage. If necessary, clean or replace it."

76k miles on original cap/rotor; still getting 28-30mpg, no engine problems. I am doing timing belt replacement in a couple weeks, and have purchased new cap/rotor.