Just as important as time is the temperature you maintain. If you keep it at a sub simmer you are left with a thin flavored clear stock.
The trick is to boil a little harder than you’re used to - that creates a thick cloudy stock like you see in Japanese tonkotsu ramen or Korean gomtang. Obviously this is a taste preference thing, but if you’re really looking to get the most out of your bone this is the way to do it.
You can actually use the same set of bones and extract different flavor profiles out of it. For example you can do a first stock at sub simmer, say give it like 4-6 ish hours and you’ll get the clean stock. Set that aside and add water again, and boil harder for 6-8+ hours. You’ll get a nice milky stock this time.
If you’re using thick bones like beef marrow or ham shank you can take it for another 6-8+ hour boil. I like doing it in stages like this because I feel like it gives you distinct tasting stocks with different characteristics that hasn’t lost too much to extended vaporization.
YMMV depending on the bones you use.