The drakes should be about 11-15 lb., ducks 4-7 lb. live weight to be mature. I think for young birds, 12 weeks and then again about 20 weeks are your window of opportunity. Unless you plan on skinning them you want to make sure they are not growing new feathers. I would catch at least 2 and inspect their breast feathers carefully. Run your fingers through them, if they feel rough, bumpy or you see shorter feathers than the full ones wait a few weeks.
Ducks are a lot of work to pluck and the more you can help yourself the better. Enjoy, Muscovy are delicious.