If it's something that I absolutely love....I will buy at least 4 or 5 yards of it. If I am buying to make a certain quilt, I usually buy a yard more than what the directions call for. Especially if I am doing a fat quarter quilt, I will double the amount in case I like how it comes out, that way I can make another.
I have bought fabric that I liked, got home and decided I wanted more, only to go back to the store to see that they are sold out and aren't getting anymore.
Hence, why I buy MUCH more than I actually need.
If it's a GREAT sale that is going on, I will buy the bolt, of course, that is IF I can afford it. Since I am not working right now, I am given an allotment each month for fabric buying. Most of the time I will go over, but we won't tell hubby ;)
I will buy companion pieces if I like them, but most of the time I don't. Just depends on the mood I'm in, I guess......