I really like scrappy quilts and that's what I make the most of, so I almost always only get half-yards of fabrics when I'm buying on a whim. One yard if it's something I think I will use in more than one quilt; and rarely more than that, although I've gone all the way up to 10 yards when it's something that see that I get that "MUST OWN LIFETIME SUPPLY OF THIS!!" feeling about. (There's only been two fabrics I've bought that much of on a whim though, and I still have several yards of both left and don't regret buying a single inch of either.)
One of the LQS here sells half-yard precut batiks...ohhh that table is SO HARD to walk away from!! I want them all! I don't think I'll ever get tired of batiks.