My kids all sleep with down comforters on their beds.  They all have duvet covers, but I started to think about and wondered how often down comforters should be washed or even replaced due to allergens and dust mites building up.  Any suggestions?

The duvet covers should address any issues, but if you are concerned, I think you could put the comforters through the dryer and that heat would kill dustmites, etc.  I hate washing down.  It's really hard to dry it appropriately.