The short answer: anywhere from monthly to twice a year, and the reason the published advice spans that range is that three things change the answer. Whether the thing on your bed is a comforter with no removable cover or a duvet insert inside one. Whether you sleep under a top sheet. And how often you wash whichever layer sits between you and the filling. A comforter used with no top sheet and no cover wants washing every four to six weeks. A duvet insert inside a cover you wash weekly needs it two or three times a year. Same bedding, same hygiene, sixfold difference, because the question is really about which layer takes the sweat.
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