The feed to meat conversion slows considerably after 6 weeks. I have weighed and graphed two batches so far this year. The growth line is very steep up to 6 weeks and begins to become less steep after that. The last batch of 51 averaged 7.14 pounds dressed at 60 days old. (a few pullets in the mix brought the average down) but many of the roos were well over 8 pounds. I've decided that an 8 pound chicken is just a little too big for my needs not to mention that cost per pound goes up after 6 weeks of age. If you like a 4 pound bird, 6 weeks is the time to put them in the freezer to get the lowest cost per pound. If you want a big roaster, wait longer, but understand that the cost per pound will increase.