I have no rules but generally buy 2 yards (I generally use a sheet for backing that I get at Goodwill). Today I bought burgundy paisleys to make a log cabin at my quilt retreat. If I run out of fabrics, I'll make a border of a solid fabric. I spotted batiks on this website for $6 per yard and beautiful 100% cotton fabrics for $4.50 so I may buy more at those prices.