It depends - on what type of fabric it is, on how much spare money I happen to have, on what the price is. Usually, not more than a yard of a focal-type fabric or novelty print. I make a lot of small quilts, and I'm still really figuring out what it is I like (too many times I've ended up with fabrics that I later hate). If I love it but it's a novelty print that has very limited uses, I'll just get a fat quarter. If it is something like the very nice quality, cute fabric with princesses that I know my daughter will love, and the price is $3 a yard, I'll get three yards. But I think that will end up being several small things - apron, a cute top, maybe a pillowcase. But usually, I don't get that much without a project in mind.
I do a lot of small quilts and scrappies, though, so I don't feel like I usually NEED yards and yards of any one fabric. If I found a really good dark blue background-type fabric on sale, though, I'd buy like ten yards if I had the money, because I know I'd use it, but I never find anything like that.