It depends on how big you want them to be. They grow slower than other duck breeds and won't reach their full potential until 6 months or older. They also feather in a little slower. Typically, you want to process ducks after they are fully feathered in which in muscovy is 14-16 weeks. I've butchered both full grown, 7 month old and 16 week old muscovy. The younger females that I did were almost not worth it. The breast is maybe 1/4 lb, the males breast probably 1/2 lb.
When I did the 7 month olds, all drakes, the breasts were at least 1.5 lbs each. The same with the year old, though he is still in the freezer.
They take alot more food to get to a good butchering size, in my opinion. When I butchered the 16 week olds, I also butchered a same aged Pekin drake. The muscovy were ALOT bigger than the pekin but he could have been processed at 8 weeks.
I am currently raising some pekin females with a muscovy drake to see how quick the crosses will grow. One female started laying a few weeks ago, so I'll have to see.