There is a really good local market near me that has fresh fish (well, as fresh as you can get in the desert) and they always let me take home the carcasses. And they have shrimp too--I love shrimp stock. Mmm, I want to try lamb stock too!
6 hours is about my maximum too, in fact I'm staying up right now studying and waiting for my stock! It is just so much fun and so tasty and economical, especially if you use the congealed fat for cooking and use the leftover meat in salads and other recipes. Plus it's helping me get some variety back in my diet that was making me insane.
Your beef soup sounds delicious!! When I was vegan I'd make cream soups using cashews, onions and broccoli which came out pretty well... and I'm working on a pasta-free minestrone, with extra cabbage. I like substituting Jerusalem artichokes for potatoes.