If it's a fabric that I don't think I can find more of later - something really special - I buy at least 2 yards. It often depends on the price; if it's a real bargain, and I just love it, I'll buy at least 2 or the remainder of the bolt.
I made one crib quilt using a fat quarter bundle, but ended up having to buy more of two of the fabrics (fortunately they had more), and didn't use any of one fabric because it just didn't work well with my pattern. I don't think I'll buy fat quarter bundles anymore. They just weren't the bargain I thought they would be even on sale.