I think after a while you get a sense of what you might use that fabric for someday..or it is the perfect addition to fabrics already in your stash..my general rule never less than a yard, more commonly 2 yds. thinking of borders or backings, and batiks if i really like them or they are not usually found other than online at least 3 yds..this is what i probably do most often when buying fabric for "piecing stash" backing stash is a whole other topic...lol