A lot depends on the size of the bones. Smaller bones= more surface area= less cooking time.
A common technique is to employ longer cooking times for stocks with large bones, then "refresh" this stock with fresh meat scraps or fresh bones for a better flavor.
Bear in mind the long simmering times (Boil is a dirty, nasty, unspeakable word) are mainly to extract gelatin from larger bones. You can also get natural gelatin from turkey/chicken wingtips or split and blanched pigs trotters or calves feet. These items are usually dirt cheap and provide more flavour than just bones.
Another technique is t do a "remouillage" , that is, once you strain off the stock, you fill up the pot with fresh cold water and do it all over again. This provides a stock with a weak flavor, buit remouillage is more commonly used to start off a new stock with freshly roasted bones--this is a lot better than just plain water.
A good book to read on this subject is J. Peterson's "Sauces" Highly endorsed, by many of us on this site.
Hope this helps