I have never bought charm packs, layer cakes or jelly rolls. However I do buy fat quarters and yardage. I just recently started Bonnie Hunter's Orca Bay Mystery. I was determined to use fabric from my stash. Then I realized that I needed more variety in 3 of my 4 color choices. So I added some more - it included fat quarters and yardage. In this case I bought 1/2 yards except of a couple of pieces which I really liked - then I bought 1 yard pieces.
If I am just buying fabric because I like it I will probably buy 1 yard. If it is a piece that might make a great border I may buy as much as 3 yards so that I can cut the long borders as 1 piece.