Fabric for backings is a good idea. I used to buy fabric when I liked it, but with no pattern in mind. It always seemed to me that when I wanted to use it it was too small, or not right. It seemed when I finally had a good pattern to use it in it had gone out of fashion (remember all those cute little calicos?) I have a ton of scraps, so I don't whan to add to that. Now I just buy what I need for a pattern. I still end up with scraps! I am trying to clean up my sewing room, cutting the scraps into usable sizes, and I am amazed at what I have found hiding here and there.

I would think it would depend a great deal on what you usually make. I primarily make bed size quilts; and usually king size. I would think about 3 1/2 yards if I thought it would be a border. 1 yard for binding, 2-3 yards for focus fabric, 2 yards for blenders.
If you primarily make wall-hangings, baby quilts, tablerunners, or other small projects, then I would think 1 yard or even fat quarters would be appropriate.

3 minimum if I like it 5 or more if I love it!:)

In the past, I bought 4 yards of every fabric I liked. That means I have PLENTY for cutting borders without piecing, and for backings, as well as for actual quilts. Now I have a stash that is so big that I don't need to buy fabric for YEARS! I just buy occasionally to fill out a project. However, that doesn't keep me from buying new fabric - I'm still a fabriholic - one fabric at a time - LOL.

I can't tell you how many times I've walked away from great sales because I have no idea how I'll use it. Occasionally I regret it but usually if I don't I just figure it wasn't meant to be mine.

I make queen size quilts about 99% of the time If I buy 3 yds. I find I always have enough fabric to work with most quilt patterns. I buy 4-6 yds of background sometimes the whole bolt. If you are planing on stack & whack or paper piecing then you will need to follow the patten.

If it a really good buy and I will use it for back round fabric, I will buy the bolt or 6 to 8 yards

i do other sewing besides the making of quilts and quilted stuff so i always buy a minimum of 3 yds...of each fabric that i am interested in...i do this in case i change my mind about what i might need to make at a later time. i also spend a lot of time going through the selvedge and bolt ends and pieces bin.

That is a good rule to follow. For me, if the price is right and the piece could work as backing, I get a minimum of 5 yards. Alas, my (as you call it) lovely stash is full up and I cannot go on shopping excursions unless I use up some.

Because of that, I sometimes sneak in a fat quarter or two because the color tickles me. There's ALWAYS room for a FQ.

I'm a bad influence, but I say as long as you can afford it, buy as much as you can!!! If it is something I like and I plan on using, I buy atleast 5 yds of it.