I NEVER have a pattern in mind when I am looking at fabric. If I do, I usually TRY to have the pattern with me, or TRY to remember HOW MUCH I need. But, that usually never works.
Here's what I do.
If I like it and I walk back to it a couple times and say...yea, it's nice, then I'll buy 1/2 yard. That's 2 fats and I LOVE fat quarter quilts.
If it's something that will make a nice border or eye catching part of a quilt, then it's usually 2 or 3 yards.
If it's for a backing and I think it'll look nice in the front of the quilt......well, then I might buy the entire bolt, but usually about 8 yards.
If it SCREAMS my name and won't let me out of the store without it....well, that means I HAVE to have it and it could be anywhere from 6 yards to the entire bolt.
I have learned that if the fabric is really pretty and you REALLY like it, then buy at least 2 yards. I made the mistake of only buying a yard of a fabric I LOVED, only to go back and it was gone. I still have the fabric, not sure what I will do with it, maybe cut it for a fat quarter quilt, dunno, but I really LOVE it and DON'T want to cut it. lol
I think it will be one of those that stays in my "collection". ;)