There is no single answer, I have a friend and if she likes something she buys the rest of the bolt. Even if it is brand new and just put on the shelf.
When I'm just collecting fabric, typically I buy a half yard, but I have a scrappy style.
Some of it depends on the size of projects you make, for me I usually work queen sized and with the size of blocks I make/number of seams, it is about 10-12 yards of fabric for the top. So -- if I really really love the fabric,6 yards means you can do just about anything with it, including a back. 3-4 yards is very usable. Then it starts going down from there.