I killed one recently that was about 18 weeks - he was a silver appleyard type (type because he was white but I thought pure appleyard, freaky things going on there) which is a good meat bird and he was good but much leaner than those you'd buy in the supermarket. I think muscovies are bigger though - ducks tend to be faster growing that chickens to begin with but then end up smaller. I always look at them at about 8 weeks and reckon they're nearly full grown but if you pick them up they're not solid yet.