This is probably the most common question I get - you know which fabrics you like, but you don't know HOW MUCH to buy. None of us can see into the future, but here are some ideas to help you figure out your own system.
When I first started quilting, I bought half yards of everything. Lots of shops require a minimum of a 1/2 yard cut, plus that is usually enough to bind a lap size quilt, so it seemed like a pretty safe bet. If I was going to buy fabric, I almost always bought 1/2 yard.
Maybe 1/2 yard cuts are just the right size for you, but as I began to work with my fabric, I discovered that 1/2 yards are almost always too much or too little.
Now, I almost buy smaller OR larger cuts depending on the fabric. Here are my reasons.
1/4 yard cuts or Fat Quarters
If I like the fabric and will probably use it for one project, a half yard of fabric is almost always too much. Most fabrics fall into this category. For a long time I didn't buy fat quarters and only purchased 1/4 yard cuts, but I have found them to both be useful so I am no longer discriminating against fat quarters. More and more shops will let you buy smaller fabric cuts AND they will let you choose between a fat quarter and a 1/4 yard. So nice!
1 yard (or more!)
If I find a purple fabric that I really like, I buy at least a yard because there just aren't that many purple fabrics out there (and it is my favorite color). My two favorite designers that always find those magical colors are Lizzy House (who is unfortunately no longer designing fabric) and Maureen Cracknell. Maureen does a really great job of incorporating very usable, very uniquely colored prints into her collections. I originally bought 1 yard of this print when it was released, but I have bought it 4 additional times because I don't see this color very often and I use it ALL OF THE TIME.
Lots of patterns out there will ask for "background fabric." Most of the time, these are white or cream solids, so I keep a large stash of "background fabrics" in my stash. For these, I usually buy between 3-4 yards at a time and that should be enough to use them as background fabrics in 2 quilts.
(I will be doing a post all about low volume prints and building a stash to use as background fabric.)