Muscovies fatten very easily and do not require anything like mallards. However, if you have any left put them on wheat and barely for a month., it'll be a lot cheaper than layers. You may have a skinny strain of duck? Pick them up and see before dispatching, they may need a little longer. By 24 weeks there should be plenty on them if not a few weeks before. 16 weeks may be a bit premature for your strain too.
Be very aware when roasting Muscovy. It has half the fat of mallard so always add a bit of water to the pan, cover with foil(apart from the last half hour) and baste every half hour. If yours are a bit on the meagre side, roast them whole as not worth mucking about with. Hope this helps.