Seems at least monthly, just did it a week ago, but daughter left freezer open a little and now it needs it again. We will take a Marina in Key West
at Boca Chica in a few days to do washing
etc., and will do it then, I defrost by running water
from a hose over the cold plates, it's fast that way.
Plugging the drain with a cork help quite a bit, apparently every time we closed the lid it formed a vacuum and pulled moist air in through there.
A house is much, much lower humidity, and I think that is the difference