It all depends on how they are managed, we kicked ours out with the flock and made them hunt for food while free ranging with the flock, didn't feed them any different than the egg laying free ranging chickens and they still were bigger than what you could buy at the store when we processed them at 2-3 months where most of your dual purpose birds will be 6-12 months before big enough to even consider it.
We currently have one cornish x that is coming up on a year old and is laying fertilized eggs from our barred rock rooster, though it is getting harder for her to get around she still does, hopefully will have the coolerbator done and try hatching some of those eggs she lays as to get the cornish x they cross the cornish with a plymoth rock