I like to sew garments as much as I do quilts, so my stash gets kind of odd at times. For garment fabric, depending on what I think I might make (skirt, top, dress etc) I will buy anywhere from 2 to 5 yards.
For quilting fabrics, I dont tend to buy that much for stash purposes. If I love it and don't have a plan for it, I will buy about 5 yards of a neutral fabric, and 3 yards of specific colored prints. I also tend to buy entire bolts of solids in white, off white and black when the sale price is really really good.
The biggest way my quilting stash has grown over the years is something I give my husband credit for. He loves shopping ebay and getting deals. He buys huge lots of fat quarters for me. I told him I though that a dollar a fq was a good price, so he goes out of his way to get them for 50 cents each or less. He buys big lots of them, around 100 assorted colors at a time. Sometimes I end up with 2 or 3 of the same fq but not often. He also discovered on ebay that people sell lots of "scrap" quilting fabrics. The lots he is buying me tend to have "scraps" that are mostly 1 to 2 yards, sometimes up to 5 or 6 yards, sometimes only 1/2 yard. The lots he gets usually have 20 or more different cotton quilting fabrics. He likes to pay about $2 a yard for the lots, and that includes the price of shipping.
Yes, I end up with some ugly fabrics in my stash, but even those have a use. I often cut up ugly fabric to make a test block of a quilt I am thinking of doing. Once in awhile I find a friend that really loves what I think is ugly, so it is easy to make a gift of whatever yardage is there.