Yeah, I hate to say it but.....last Spring I bought one Cornish X to raise with my layer chicks. We planned on it being eaten but I was curious to see what the growth rate would be if they were raised just like the layers. "It" turned out to be a he but I didn't know that until I was processing him and found the gonads - his comb didn't turn red like other boys I've had and I thought he was a girl throughout his life. Here's what I did and the results:
He was in the brooder with three layer chicks for approx. 4 weeks. He then went outside to my chick run for another 3-4 weeks. During all of this time he had free access to feed/water. (In the brooder this was 24/7 but once they were moved outside it was only during the day and they were locked in the coop at night).
After the time in the chick run, the four chicks were let out to free-range with the big girls. For the next 2-3 weeks he free-ranged my backyard, again with free access to food/water and all the greenery/bugs he could find. At this time he was noticeably bigger than the others but a long way from full grown. I think this is because he was raised on regular chick food rather than a broiler mix, and got a lot more exercise than meaties usually do. I will say that during this time I started to notice him having trouble keeping up with the girls. They had more energy and liked to roam long distances. He often straggled behind them but valiantly tried to keep up and always caught up to them when they stopped.
One day when he was 10 weeks old I looked out side and noticed him lying very still in a corner of the yard. At first I thought he was just asleep. It was only an hour or so before that I'd been outside hand-feeding some treats and he was right in there with them all getting his share. But when he didn't move for a few minutes I thought I'd better check on him and discovered he had died. I assume it was a heart attack. Not wanting his body to go to waste (and knowing he hadn't been gone long), I plucked and eviscerated him (this was when I discovered the gonads), and we ate him a couple of days later.
So there ended the great meatie experiment for me. One thing I will say is he was the sweetest chicken I've ever had. He loved humans and is the only one I've ever had that not only didn't mind being picked up but seemed to actually like it, and would hop around under my feet almost begging for attention. I was sad when he died (ironic since I'd bought him to eat in the first place) but glad he had a good life for the short time he was with us.
This birds are genetically designed specifically for meat, not meant to live a good long life like most chickens, but meant for the table REALLY early on.
They normally go outside around 3-4 weeks, when they have sufficient feather cover so they won't get cold (depending on time of year). I then limit their feed, but don't really do a 12 on/12 off schedule. I feed them a set amount twice a day and when they run out they're out. Then they range for the rest of the day. I've never lost one due to leg issues. I've never lost one to heart attacks. I almost lost 3 to heat issues, and I was quick enough to cool them down and save them. 
