I followed Alton Brown's recipe for stock in which he gave a rough estimate of 8-9 hours at a low simmer to extract maximum flavor. At one point, he said, paraphrased, "How will you know you've extracted the maximum amount of nutrients from your bird? Well..." at which point he fetches a rather large leg bone from the discarded remains and easily snaps it in half. Alas I couldn't do that. I'm wondering if I did something wrong or if my results are typical.
I had a half carcass including wings, neck, back, gizzards, skin, and bones. They were raw and I froze them. For the stock, I gathered 3 carrots, 3 ribs celery half a bulb of garlic, and some thyme. I skipped on any salt or acids. I threw these with the frozen chicken into cold water, brought it up to a low simmer, and left it as such for 9 hours. The flavor was tremendous. Certain smaller bones became mushy toward the ends. However, I couldn't snap my bigger bones.