So how much do you buy?
Well, if you have a tendency to like certain types of fabric and you already have 5 pieces of yardgoods back in the stash that are similar in color and print you really shouldn’t buy it at all – so, if you must, get a quarter yard. That’s it. Okay, maybe a half yard if you really really love it.
That’s enough to feature in a baby quilt or accent in a bed quilt. I tend towards half yards and the occasional fat quarter. It never hurts to ask if the store will cut a fat quarter -- the worst they can say is no. [A fat quarter is a half yard cut in half. A quarter yard cut off the bolt would be 45" by 9"; a half yard is 45" by 18". You get 2 fat quarters -- measuring 22" by 18" -- out of every half yard.]
If I’ve nothing like it in my stash and I think it would be the focal fabric of a quilt project I get between 1 and 2 yards, usually 1. I don’t tend to make large quilts so usually 1 yard is enough. Unless it's my background fabric -- I always run short of suitable background fabrics so if I find a versatile light print that I adore I go for 2 yards minimum, sometimes more. Because the traditional block patterns I favor tend to need light backgrounds and often I use an alternate block setting (with plain background fabric squares) so I know that will always be useful.
Part of the equation depends on how large your quilts are going to be.
Part of it is what style of quilts you do.
Part of it is how much money you can afford to spend/want to spend.
Part of it is finding good quality fabrics on sale.
When stash shopping keep in mind:
Buy some fabrics that you might like together in a quilt, not just random odd pieces.
You want to balance large, medium, and small prints.
Don't buy all medium values -- get dark darks and light lights too.
You should always have a dot and a stripe and a check on hand. Serpentines are also good.
You can never have too many tone on tone prints.
Solid fabrics don't go out of style so those can wait until you know what you need when you're actually doing a project (unless there's a really good sale or the store is going out of business).
And don't worry if you bought a quarter yard too much, "leftovers" make great scrappy quilts.