How many grams of gelatin are in stock/broth?
This is going to be tough to answer. Too many variables. 

I pressure cook all my saved chicken bones until they can be pulverized easily between two fingers (only about 1 hour). Then I reduce the stock from 3 quarts down to about one. When cooled and poured into a can-and-freeze jar it will typically gel at room temperature. 

The answer depends highly on the amount of feet used, the amount of water it was cooked in and how much reduction is preformed.