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.
Broth=meat based
Stock=bone based
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.