Wow, I really didn't anticipate such a variety in opinions. I guess there is no straight answer. Anyway, I think there's a problem with cooking stock for more than 8 hours or so, since it can cause too much water to evaporate, forcing you to add more water. Definitely not overnight, since you can't be watching over the pot making sure there's enough water left.
I cook my stock until it tastes like the meaty flavors are well-combined, and it doesn't taste like water with floating bones. It means about 2 hours for chicken, fish and shellfish, and about 5 hours for mammal bones. I never witnessed any bitter taste to fish or shellfish stock cooked for a good 2 hours on stove.
See, I don't think a stock should have a strong flavor. In fact, what we really value in a stock is a more neutral flavor that allows it to be a carrier for other flavors. It is, infact, why we prize veal for stock making. Its abundant gelatin and relatively low amount of flavor. We want our sauces to have a small amount of beef/veal flavor, but we value the mouthfeel of the gelatin more, and the neutral flavor of the stock allows the flavorings (herbs, veg, wine, spirits, spices, etc) to shine though more. Of course, we can fortify stock with additional flavorings like meat trimmings, but if we don't know the final purpose of the stock it is better to keep it neutral and fortify it later.
If you are talking about making a flavorful broth, say for a soup or a stew, or what have you, then that is a different thing.
But again, it is hard to go "wrong" with making your own stock. Like I said, if you are simmering meat/bones in a pot with aromatics to cook with, you are ahead of 99% of at home cooks that reach for canned broth or bouillon cubes.
And again, I think is is definitely a point of diminishing returns on a stock. To me it is like a bell curve, a long time when it is developing flavor, a small window where it is "optimal," and a long period of slowly deteriorating quality.
So yes, you could simmer your chicken stock for 12 hours, but it might not be as good as it was 6 or 7 hours ago. It is still good and certainly usable, but it's past it's peak. And you guys, if you have never tried, should try adding your aromats (veg, herbs if using) near the end of the cooking time--like the last hour or so--it really enhances the aroma of your stock.
I think that a good stock should have a deliciousness effect all by itself. What a stock does is elevate a dish, and in order to do that, it has to truly spectacular flavor. The tastier a stock is, the more it adds yo your dish. The problem is, how do we know when a stock is at its peak? What should it taste like? What should the bones look like?
Adding the aromatics at the end is a really good tip. Thanks, Someday!
I like clear stocks made by simmering bones and aromatics and all that, but like a couple of you have already mentioned, I also dig that Japanese ramen broth, and also Korean sul lung tang broth. Both are boiled to death with the lid on for close to or over 24 hours, and are just so damn tasty. I wouldn't call them cloudy since they are normally not made with any veggies; they're just thicker, like how milk or cream is as opposed to water. I had this fish soup once in Korea and I had to ask the chef how he made it, because it was unlike any other fish stock. It was milky white like ramen broth, and pretty thick. It was only the stock and some seaweed in it. The chef said he boiled - not simmered - fish bones for a long time. I think it was probably the most memorable fish soup I've ever had.