In my opinion, for any type of plug, removal and replacement with a new thin coat of anti-seize compound should be done once a year regardless of mileage. The reason is to eliminate the probability of the plugs "freezing" in the aluminum heads if left too long. The resulting agony is excruciating. Platinum plugs are sold on the basis of lifetime thus discouraging the buyer from removing them periodically.

If you take my advice to remove the plugs from time to time, you may as well clean and set them and also inspect them for quality of combustion. I would (and do) use the copper plugs for that reason.