It depends on the fabric and the cost. Blenders, 1.5 to 2 yards. Anything else that I really like or can visualize in a quilt, at least 2 yards.
I've gotten more of a fabric if the sale is ridiculously low priced ($1/yard), and have started donating some of it to my guild for charity quilts.
Even so, my stash is starting to take over the house!