For the one in the house - Every other Friday, by the maid who takes most of the "leftovers" home with her.

For the one here in the office - Whenever it needs it. Which is not too often.

For the two in the hangar -
One is nothing but sodas, fish bait and aircraft parts requiring cold installations, so not too often.

The other is nothing but beer and other such beverages, so even less often.

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1980 300D - Veggie Burner !