You build a stash by getting OLD! :-) At least, sometimes it feels that way when I look at mine and ask myself "how did I ever get so much fabric?" Not complaining, mind you, but do wonder at times.
I agree with the others about yardage. If you look at some of your patterns, they'll give you a general idea of what is required in terms of the largest amount you might need, depending upon the size of the quilt.