The barred rocks I raised took between 12-16 weeks to come up to between 3-4 lbs. They have narrow bodies and not a lot of breast meat but nice big leg quarters. They're really tasty. Make sure you feed them 20%+ broiler feed.
I fed mine flock raiser as they were in with my hens until today. Today they were processed (my first time) at just a few days shy of 20 weeks. There is a pretty good amount of meat on them. (just don't expect breast size like the grocery store) I considered butchering them at 16 weeks, but they just didn't look big enough to me. In the last couple weeks they have really filled out.