As a rough guide, I figure 10-12 yards of fabric for the top of a queen sized quilt. Backing is another 6 yards (minimum) or 3 yard cut of a appropriate wide width.
Sure, you can make a quilt with less than that. Or more. Tiny triangles can have almost as much fabric in the seams as shown on the front.
So when you are confronted by an absolutely magnificent piece of fabric and don't know what you want to do but that you must buy it, think of it in terms of proportion of 12 yards. If it is going to be background or main focus, 6 yards. I mostly buy 3 yard pieces but I'm trying not to buy new fabric any more, working out of my stash. I do shop thrift stores as something different than "new" fabric
And remember, it's better to have half a yard fabric too much than 2 inches not enough!