When in comes to hanging mallards, most of my UK friends recommend 3-4 days, but with the semiaquatics, I pluck and clean them the same day and age them in the refrigerator for a day or two before cooking.  The mallard is an opportunistic omnivore and, therefore, it is a bit of a lottery in terms of taste from bird to bird.  You can plan for a half bird per person, if serving wild mallard.  Plumper, younger birds are perfect for roasting.