Until it looks like soup. If you're using a whole fresh chicken, after about two hours it will start to slide off the bone and there will be enough flavor in the pot. If it's carcass soup, you may want to let it simmer longer, till you have total meltdown. Save it for when you need it. Freeze some soup in one-cup containers for recipes that call for soup as an ingredient.
Some cooks make soup cubes in ice trays.