I buy at least 3 yards if I really like it and at least a yard if it appeals to me.
Sometimes when I buy collections I will buy more of one fabric so it can be the focus and then 2 or 3 others of lesser yardage--so they can work together real good.
That's why I have tubs and tubs of fabric.
Most of the time I buy because I like it and not because I have a project in mind.
Chris