it all depends on how much you feed them.. if you give them as much food as they will eat 24/7 they will grow faster and get to butcher size at 6 to 10 weeks (depending on if you want fryers, broilers ect.).
however if you restrict their feed they will grow much slower and will not be at butchering size at 8 weeks
many people who want to try to keep some of the cornish X as pets or as future breeders (yes I know they don't breed "true" but there are some people here who have added them into their breeding programs) will restrict their feed and allow them to have more exercise (free ranging) to help keep stress off their joints and heart. Their birds do not grow as fast as someone who is feeding them for growth and to get them to butcher size in the standard 8 weeks.
I know there are several threads here on the subject