I buy three yards if it is a focus fabric so that I can make sure I get legthwise, one piece borders out of it, and use the rest in the quilt, that will 'eventually' get made.
I buy two yard pieces for baby quilts, so that I have enough for a backing and can use the fabrics in the blocks as well, or for borders and blocks.
If the fabric will be fussy cut to get a specific area of the fabric for a center block, I count the number I will need and a few extra, and then buy the amount of fabric needed.
If it is a blender or sale fabric that will make a great backing, then it is 6 yards. I have hit some great sales on Air Force fabrics that were not great for use in quilt blocks, but made awesome backings.
I only buy fats if I can get 4 alike, that will make great accent fabric for something I already have, or to make purses or totes (sometimes 4 fats are cheaper than 1 yard during a sale). I especially like fats to make 'trip around the world' quilts, and make sure I buy at least two alike. And lastly, I love Christmas fats to make sampler quilts.
I buy Kona cotton by the bolt when I can with a 50% off coupon at Joanns.
I try very hard to not buy fabric just because it is a great deal, but because I can really see a quilt that will come of it. Quilting is just getting so expensive!!
Right now I am collecting Christmas blues and silvers to make the famous Dunster's Diamond Star Log Cabin quilt, in one yard pieces. Watching all sales year round to get the best deals.
Diane