I think what you buy should also depend on the type of quilts that you like to make.
I enjoy making samplers, so if I see a fabric that would be a good "primary" fabric for a quilt, meaning for me, I'll use a little in each block and use it for sashing and borders, I'll buy 3-5 yards of that.. if it's something that will make a good secondary fabric anywhere from 1/2 to 2 yards.
Fabrics that I find that are a bit more "neutral" I may buy enough for a back and binding.. I rarely buy less than 1/2 yard.