There is indeed a huge range of time you can settle on for the broth. The longer you let it settle, the more intense the flavor will be and the more nutrients will be in the resulting broth (and the less nutritious the bones and remaining meat will be, as well). 72 hours is the best time overall if you can spare it. However, going for at least 20 hours or so (or even a full day) will also result in a delicious and useful low FODMAP bone broth for your upcoming recipes.