Ok so basically I have been going through some books lately and I quite confused on this topic. Basically its about stocks and sauces, some chefs recommend that you cook them for a short period of time to not "lose" and of the freshness then I find recipes where sauce/stocks are cooked for hours.
Everything depends and is unique. I do fumet, as lagom said, no more than an hour, starting with ice and never boiling it- making more of a fish tea. Chix stock takes 6-12 hours, and it takes three days to get proper veal stock.