How much to buy depends upon the style quilting you do. Scrappy, matchy/matchy, art. I like scrappy and art so usually buy 1/2 yd. Then if it's a really good price, buy the back-the rest of the bolt.
From the time I started quilting to now, my style has evolved into what I now call "my voice". I regret spending so much money in the process and for big projects. I only have 2 beds in the house and my daughter has gotten more than she can use. Now I use those early purchases for donation quilts