how long does a duck have to hang for...?

Once you have cleaned the innards out and washed the cavity, let them hang for 2-3 days, this allows the flesh to relax and the feathers come out better, even use an old shaving razor to remove the pin feathers and little bits of fuzz. You can soak them in a bucket overnight in a salt brine with a few onions and carrots, this removes a bit of the wild gamy flavour and as wild fowl eat alot of aquatic materials the fishiness too.

As a chef I only did this a few times, one time at a hotel I worked at here in Canada, we got a load of wild ducks, snow geese and other wild fowl for a First Nations dinner function, we had to clean some of them, like regular poultry after there cleaned you can dip in a pot of hot water and scrape with a knife, like the razor removed pin feathers and any other bits you missed, then when cooking as wild game and fowl are very, very lean, I would recommend using rashers of bacon or thin sliced pork fat over the whole bird or breast meat to aid in cooking these can be eaten or removed, also do not overcook them as they dry out fast, in traditional french cuisine the method of cooking them and other birds is called "poelle" which is a constant basting of butter and/or fat and broth in a covered pan or casserole, slowly cooking them until done.

funny....ha ha ha...... really,no need to hang...