I got some Cornish X Rocks that were supposed to have been butchered back last December, but for some reason didn't seem to be growing very well, and they stayed small for the longest. Then one thing or another got in the way, so we didn't get around to butchering any of them until the weekend before last. By then, they had been on a diet of turkey feed (28% protein) and had muscled out really well. My smallest dressed was 5 1/2 lbs., the largest a little over 6 lbs. These birds were right at 6 mos. I don't think they would have gotten any larger than they did. I still have 5 more to butcher out next weekend. They were so large, I ended up quartering them into cuts rather than freezing them whole.
Layers will have next to no real muscle on them. They really aren't worth the trouble to slaughter, IMO. We did that with our extra roos, and were pretty disappointed. Layers are putting most of their resources into making eggs, not muscle. The Jerseys were the biggest disappointment. They do take longer to mature, the first 6 mos. going into growing their bones. I don't do it for mine, but I would think feeding them a higher protein feed might help.
Dual purpose birds are a little better, but still not overly meaty. I had to cull some extra BLR Wyandotte roos, and they definitely had more muscle, but not as much as I would have liked.