I buy according to what I think the fabric might end up being used for. If it's a great border fabric get 3 yards. A great background fabric get at least 4 or 5. Anything that will be used for blocks, anywhere from fat quarters to yard cuts. If you are going to buy a great focus fabric and two or three fabrics to go with it for the blocks get at least two yards of the focus fabric.
A queen size quilt averages around 10 yards for the top.