How many times can you reuse bones to make broth? I recently cooked a chicken and made chicken broth from the bones. I also kept a few pieces of meat to add to the broth to give it more flavor.

I know you can freeze the broth and use it for later but specifically how many times can I reuse the bones to make broth? Is there a time when it loses flavor or is not good for you? Does it differ depending on the animal/meat?
Beef bones can be used multiple times, but less flavor and gelatin will be extracted from each additional use. Harold McGee's "On Food and Cooking" describes this. 


  Because a standard kitchen extraction of eight hours releases only about 20% of the gelatin in beef bones, the bones may be extracted for a second time, for a total of up to 24 hours. The resulting liquid can then be used to start the next fresh extraction of meat and bones.


I would imagine chicken bones would be similar, although, since they are smaller, the first batch may be more effective than 20%.