Well, I joined the Fabric Moratorium because I am trying to reduce my fabric. I have given myself an exemption for Thrift Store fabric which is how I usually buy. It's $1-2 a yard typically and I have to take all of any given piece or I may have to buy an entire bag just to get what I want (bags usually are $4-10). Since you can't go out and find what you want I have to "shop ahead". But so far this year for "new" fabric I've bought a half yard of a print for a specific project and I bought about 20 yards at Walmart for $1/yard. BTW, one of the fabrics I got there is on sale for $8.50 a yard at an on-line shop right now...
But for fabric stores... it depends on your budget, your space, quilting style, and other things -- like sometimes we just gotta have it without the slightest idea of what to do with it. For me, I don't buy cuts under 1/2 yard because I've been the clerk before. 1/2 yard for me is a lot of fabric, but it isn't very usable for other people.
Just as a starting point, any given bed size quilt will take 10-12 yards of fabric. So I consider 3 yards a very generous piece able to do most anything I want. If it's going to be the background, then yes, 5-6 yards is reasonable.