Jumbo Cornish Rocks get to be full size at about 6-8wks. of age. I've heard that soon after about 8 weeks of age they begin to have trouble walking because there bodies weight becomes too much for their legs to handle. I hope this helps!
We had two batches of cornish cross last year. The first batch dressed out at 7 lbs at 9 weeks. The second batch barely weighed 5lbs at 8 weeks. I am raising two right now as an experiment to see if fed like a regular chicken how fast they grow. Currently they are almost 2 weeks and they are only a little bigger than my sliver laced wyandottes.
We give ourselves at least 8 wks for cornish x. Not sure for the pekin duck. You might want to ask in the duck section.
