Generally, you should aim to wash your comforter every one to two months but treat stains and spills immediately. Monthly washing helps prevent reduced loft from soils building up inside the comforter on the filling. How often you wash your comforter typically won’t change based on the type you have (down, down alternative, etc.), but you should always check the manufacturer’s instructions when caring for your specific comforter.
If your comforter has a duvet cover, it can be washed less frequently. "Since the duvet insert doesn’t make direct contact with skin or your bedroom’s environment, you don’t need to clean it as often. Make a note to clean your duvet insert every one or two years, more if you store it without a cover for any reason or if you notice damages or staining.
Top sheets can be a divisive subject—some people swear by them and others see no use for them. If you don’t use one, you will need to wash your comforter more regularly. "Comforters used daily without a flat or top sheet should be washed weekly just like the bed sheets and pillowcases. This is because body sweat, skin flakes, and oils are going right onto the comforter rather than touching the flat sheet.
Always wash a comforter when you first buy it. "While it may not be visible, traces of chemicals may sometimes be left over from the manufacturing process. Additionally, comforters may collect other irritants during packaging and/or transit, which can cause itchiness and discomfort.