Just as airing your dry duvet outside has its advantages, drying your duvet outside after washing is the best way to keep it feeling and looking fresh. How long you air it out will depend on the type of duvet, and whether it's dry, or not. When airing a dry duvet out according to the Scandinavian sleep method, you should leave it out there for four hours minimum. The longer time you have the better, but four hours will be long enough for it to fluff up and de-moisten. If you've washed your duvet and need to dry it naturally, it can take much longer, especially if it is made from down. A freshly cleaned duvet should be dry to the touch after a full day outside.