I never buy less than 5 yards of a fabric, unless they don't have that much and I like the fabric, I buy what is left. If I really like it I'll buy 10 yards.

I have been know to purchase an entire bolt if the price is right! (Like $40/bolt!! - nice price for 15 yards) If I think I'll use the fabric for a backing, I'll buy 10 yards; if it is a blender I can use in many quilts, I'll buy 5-8 yards; if I just like it, I'll buy 1-3 yards.

I try to figure out where it most likely will be used (borders, sashing, backing, or blocks) and buy according to the most likely size of quilt I would make. Most of my quilts are lap size and most patterns I make need at least 7 different fabrics.

I, too, would buy three yards. When I buy fabric,I also cut a 2.5 inch jelly roll strip and save it as it helps to build my strip stash.

If I love and can afford it, I will buy 2 yards. If I love and can't afford it, I just buy one yard. But I mostly look for sales and can usually buy 2 yards at least.

I started out buying 1 yard, but now find that I usually need more to make a quilt. So I have been buying 2 1/2 yds, but I really should buy 3 yards like most here suggest. That is usually what I have needed and not had. I like to make mystery quilts, so 3 yards should be enough for most of them. I tend not to buy plains because I can always find what I need at a LQS that will work.

In this modern economy, I would opt to buy only what I needed for a project. I do have a stash and what ever I pull to use in a project is, more often than not, not enough.

I stopped buying fabric and now spend all my time now, using what I have. If it only makes scrappy quilts, so be it!
I love scrappy!
peace :D

2-3 yards if I love it.

i only shop sales, in person or internet. if i shop internet, i shop for no shipping, even if i have to spend over a certain amount.
when i shop sales, i shop with nothing in mind and i buy 3 yard pieces (remember, on sale with no shipping). if i have to wait , i do. but i don't buy backings until i need them. i shop the backings on sale also, but since i need 9 yards at a time, i wait until i need 2. then i wait for sales and only buy 2. i don't build a stash of backings. too expensive. the regular fabric affords flexibility for different designs, but the backings do not. i don't use sheets because i want the backs to reflect the tops.

EDIT: there's a fabric store in denver, pennsylvania, that used to have sales on their loading dock. they sold bolts that way. the bolts were so inexpensive that when they ran the sale, i would drive out and buy a few bolts. i went, i would say, every 2 years, and stock up. the store is sauder's. they have no email or mailing list so i don't know when or if they still have the sales. does anyone here know? please let me know.

Sauders sounds like a great place buterflywing...I did find an address and if you page down there is a nice long critique of what they stock and other good info.
Wish I lived closer though.

http://www.yelp.com/biz/sauders-fabrics-denver