If its at the quilt shop in town to I buy a yard or two because I can always go back and get more.
If its outside of town and I find that one fabric in the place that I can't leave without then I'll try to get at least three if I can spare it. If I find more than one fabric that I can't leave without then I'll try to get as much of each as I can. A yard is the bare minimum. I don't buy fractions of a yard unless I'm testing out something or have a smaller project.