It depends on what size of quilts you like to make too to figure out what you generally need to have on hand. Most of the quilts I've been making are bed sized, so I know I need a minimum of 8 yards for a back, so if I run into a sale opportunity, I stock up. Once you start cutting fabric to piece a top, your fabric requirements go up exponentially so I find about 12-13 yards of coordinating fabrics is often what I end up buying for a large queen or king quilt top and binding. Of course I have leftovers, but then I have enough for coordinating pillows or small projects, or to put into another quilt top.... I also buy bulk backgrounds, I.e. Solids in neutral colours. I don't live close to a LQS, so when I get to a shop with a sale, I stock up, so I can usually make something when I get the urge ;-)