When I buy just because I love it I usually get 1/2 yard or a yard. If something is on sale for a good price and I think it looks right for a boarder or backing I will get at least 2 yards for a board or 4 yds for a backing fabric. But that is only if I think I would really use it as a backing. Other wise I wait to get the backing when I have a quilt in mind.
I think onebyones suggestion is also very good to [have a good stock of solids and neutrals. ] and blenders.