It depends on how rich you want the stock to be, the longer you cook the richer it will become, and flavorful in my opinion. I leave the stock pot on the burner as low as it will go (that could vary depending on your stove/stove top). I have a burner that heats low enough to be able to do that.
My beef stock I do at least 3 days, chicken maybe one and ahalf. Haven't done the fish stock, but do make Bonito broth, which is much easier!
Some smaller butcher shops can order the feet for you. Also check Whole Foods, they may be able to order, I know I've gotten marrow bones there. I actually saw a grocery store in Phoenix "Tradiciones" (I think it was called) that is a restaurant along with a grocery store that had tons of chicken feet and calves feet. Next time I visit, I will bring a cooler with me!