Good Morning Craftybear,
In my quilt club, there is a very experienced lady who has a wonderful eye for color and her small quilts will cover a king size bed. Her rule of thumb is if you see something you like, never buy less than 3-yards!
If you like a certain line of fabrics, buy at least 5-of the co-ordinating fabrics and not less than 3-yards of each. You may not use them for two years (or in my case, longer) and if you do not have enough to work a project, there is almost no way of matching what you have.
Hope this has helped. Have a great day