Well,..... I like to buy stuff on sale. Fortunately I am in an area where I have a lot of Choices of Quilt shops carrying great quality fabrics. I do occasionally buy stuff not on sale if it calls my name. I will usually get a yard sometimes a yard and a half. If it is on sale, however, if it a color I like and I know will go with others in my stash, I will buy a yard min, many times 3 yards and if it is a great sale.....6 yards to the rest of the bolt. I have a bunch, about 4 large Rubbermaid totes as well as the upper left cabinet is all over 5 yards. Many pieces are 10 yards cause they were good quality fabrics that I got for $3 or under a yard.
If it's something that I don't like and I need for a project I will only buy what's needed or a fat quarter. Mostly I buy at least a 1/2 yard.