There are several of the campaign missions that I've completed already and its been awhile since I've gone back to do any of them or would you rather finish campaign in general but I recently went back and covered that some of the places I've already destroyed have restocked on elixir and gold. How often does this happen?
The loot in the single player campaign does not replenish. The reason why you can see some loot available is because you have cleared the stage before the update of April 2015.
That specific update had a revision in the loot amount from certain campaign stages and for stages where the loot is increased, the additional loot becomes available to a player who has already completed them (and new players will have the updated loot available).