I usually have a project in mind, I figure about how much I am going to need

One metre if I think it will be a focus fabric, and varing lesser amounts for the stuff to "go with" something else or to add a solid to my stash. It works for me.

I started out buying a yard of each of the fabrics I liked. Now I'm up to anywhere from 5-10 yards. I figure that way I could use it on a few projects or have it for the backing. I used to put all my ugly fabric on the backs, but now I like to match them.

I'm like this, too and totally agree. Also have to add, if I am collecting a particular color (right now it is pinks) I will buy 1/2 yard.

at least 2 yards more if i really like it

It's really an individual thing and depends on what type/size of quilts you usually make.
But I've never regretted buying too much, but I have regretted buying too little.

And if you buy too small of a piece, you can't use it for a focus fabric, or a border or much of anything except scrappy or applique. So if you like, buy lots!

I have to say that I have way too much fabric but I am always sorry if I don't buy the fabric I love. And if I love it I usually buy three yards. However there was this perfect green leaf over all print that I bought three times because it just went with all the quilts I made in that time period. They laughed at me at the quilt shop and would always go get that bolt and put it on the counter. They knew I would buy more of it. And I used it all.

I have learned not to walk away from fabrics that really grab me, even if I don't have a plan for them at that moment. I agree with the many people here who said they buy 1-3 yards of one you just love.

Usually I am buying with a project in mind so then I know how much I need, though I sometimes buy extra to make a pillow or two to go with that quilt.

I think it depends on what kind of quilts you make=
Bright solids for childrens quilts -1 yard of assort. colors
childrens prints 1-1 1/2 for piecing/borders
1/2-1 yard misc. prints for scrappy quilts
tone on tone -background fabrics-2 yards of several fabrics
backing fabrics for crib/lap quilts-2 yards

I buy everything from charm squares to yardage-if I'm in love with the colors/pattern I buy as much as I can afford. If it's a collection, fat quarters, after that it depends on the fabric and colors. I have a cupboard full of awesome pieces that I have no idea what I'm going to make with them! But I have passed up fabric I loved then couldn't find it when I went back. Eventually you'll figure out what works for you. Since I love applique and making I Spy quilts, I can use really small pieces.