What "breed" do you have? The commercial-type Cornish x Rocks do indeed grow the fastest and probably should be fryer size after 8 weeks, but there are several other broiler "breeds" (as you probably know they are all crosses and not actually breeds) designed for organic, free-range, or otherwise less intensive production that don't grow as fast but achieve comparable results, it just takes them several weeks longer to do it. For commercial operations where everything is based on economic efficiency they aren't well suited because they grow slower and eat more, plus many also have colored rather than the preferred white plumage. But they are also able to forage and have fewer health problems because they don't grow as fast. If you have one of these types (Kosher Kings, Red Rangers, etc., etc.) it may take closer to 12 weeks before they are fryer size. But in my personal opinion you will also get a better product! A friend gave me some commercial broiler chicks out of his commercial chicken house once and they certainly grew fast, but they were also the closest thing to a vegetable that an animal could be. They ate, drank, and lay around, that's all. You rarely saw them walking around or attempting to forage.