Since it is tucked under a cover, only wash it when it’s soiled. But wash the cover once every two weeks. To avoid clumping up the duvet’s filling, experts recommend sending it to the Laundromat.
At sub zero temperatures, no bacteria can actually survive it. So unless you’d spilled something in it or smell something odd (that’s not fish or durian), skip cleaning it. Normally, you only need to defrost and clean out your freezer once a year.
The steam functions practically sterilise it. Leave it alone unless you detect a bad odour. If so, you’ll need to give the machine a run with special formulated detergents before using it again.