I'm making stock from turkey bones and all of the other leftovers hanging around the refrigerator. Since the cooking time varies quite a bit depending on cooking method (I'm doing a slow simmer on the stovetop) and bone type, how can I know when my stock is done cooking so that I can begin to chill it?
The absolute time a stock will take depends on the quantity of stock you are making and also on the ratio of solids to liquid with which you started (along with the strength of your burner, the starting temperature of the water, the geometry of your pot, ...). Intentional variation is also possible depending on your intended use.
The best advice I've heard comes from Judy Rodgers in the Zuni Cafe Cookbook: taste the stock often as it cooks, more and more freqently as it starts to taste good. When it doesn't taste better than the last time you checked, it's done.
Really it's up to you. I generally cook mine for 4 hours, but you can cook it longer or shorter. Cooking it for a short time will lead to a lighter, less flavorful stock, and cooking for longer leads to a darker, richer stock.
Once again, there's no right answer, but I believe 1.5 hours and 6 hours would be best.
It depends on what is in the stock pot. Thick bones like beef joints may take all day. A simple vegetable broth can be had in 30 minutes.
Generally, for chicken stock (which is what I make at home the most), when everything is falling apart and the bones are kind of bendy, you have gotten all you can get. This takes about 3-4 hours at a slow simmer depending on the size of chunks you have put into the pot.
OF course, as the other answer indicates, it also depends on how patient you are.
Turkey stock/broth? The bones have given up all their goodness when they're soft and the smaller long bones can be bent, like the thinner bone in the wing, analogous to the human radius (yes, in birds, the radius is the smaller of the bones). When the bones are bendable, the marrow has given up it's goodness, particularly if you have cleaved the larger of the bones. At a low simmer, that can take 10+ hours.