For me it always depends on how much money I can spend shopping and how many fabrics I like so I divide it up accordingly to get some of all that I like anything from 1/3 cuts to 4 yards.

I make scrappy quilts, so fat quarters are good too. For basics like tone on tones and solids I tend to buy more for backgrounds. I prefer tone on tones over solids, I think they add more to the quilt without standing out like solids tend to do. I used to buy fabrics with no thought to what part of the quilt they would play a part in. Before fabric prices became so expensive, I'd buy 3-6 yards of a fabric.
I have been quilting since 1988.
Sharon

When a piece of fabric jumps into my arms, and I have to have it, I buy at least 3 yards, that should usually be enough for a quilt; however, when it comes to background fabrics, white, beige, grey, black, etc. I tend to buy 6 yds. if I can so that I won't run out. If it's a really good deal, I've been known to buy 6-9 yds., which would give me enough for a backing. I once ran into a fantastic sale at a LQS, who was selling from bolts of fabric that had been on their shelves, they were just making room, in that case, and I was able to buy it I bought 3 or 4 pieces of 6-9 yds. One background fabric I bought decided that it wanted to be on the front of the quilt, so I was glad I had purchased it as using it for the focus fabric in a Jacob's Ladder quilt, my church made about $500 from that raffle quilt. I have to be a little more frugal now that we are retired on SS, however, I am so glad I have my stash so I can continue to make quilts, buying usually fabrics for backgrounds and backings when I've run out of a particular color.

I will usually buy a fat quarter or 1/2 yard when I am loving it but clueless as to what to use it for. When totally gaga I buy a yard..as a result I have way too much fabric that goes with nothing else

As you can tell from all the comments there are several factors to consider: for me, budget and reason for the shopping trip- when I'm going fabric shopping for a project I try to allow at least a little bit in my budget for a gem I find that isn't for my project or shopping list. It also depends on the types of things you are currently interested in making and whether or not you've started a list of either definite future projects or at least one you think you want to make. I don't normally make bed quilts, I focus on wall hangings, lap or throw size quilts, bedrunners, table toppers, pillows, baby & kitchen items - so 3 yds would usually be the most I'd purchase. This would be for focus fabric, backing or one I think I could use in more than one project. If there are patterns or a type of project you know you want to make take a picture of it and the fabric requirements to consult when you're shopping. Enjoy!