There's no easy answer for this that I know of. A quilt that uses two fabrics in large patches might take 3 yards of each fabric while a quilt that uses the same two fabrics in very small patches could require five or six yards of each.
If you're using six or eight or twelve fabrics, it becomes even more complicated - and if you're going for a really scrappy look, you might just want to buy 30 or 40 different fat quarters.
The only way I know how to figure this out is lots of math.
If you draw a full-size template of your block, you can figure out the number of square inches of each fabric in the block. (include the amount of inch-age in the seam allowances.) Multiply that by the number of the blocks that you will need for the quilt to get the number of square inches of that fabric that you will need.
Divide that number by 1515, which is the number of square inches in a yard of fabric measuring 36 X 42 to find out how many yards you need.
If the answer for that fabric is 1.119, for instance, rounding up to the nearest quarter yard would mean you need 1¼ yards of that fabric.
Go to the next one - say the answer is .287 - you would buy a half-yard of that fabric.
Or you could divide the square inches by 378, which is the number of square inches in a quarter-yard of fabric.
It's cumbersome and tedious - if you're good at math, maybe you have a way that's easier. I'm not any good at it, but I have a calculator, so I can plod along when I have to. :)
You could instead think in terms of fat quarters and go for an estimate that's a little bit closer than a WAG. :) If you arranged 30 fat quarters in a 5 X 6 grid and sewed them together, it would measure roughly 106 X 108 - just about perfect for a king-size bed, if all you were going to do was sew fat quarters together.
But, by the time you cut it into patches, you've probably got just about the right size for a queen bed.
So, okay, 30 quarter-yards equal 7-1/2 yards. If you're using only two fabrics in roughly equal amounts, you need 3-3/4 yards each. (I would buy four yds, myself, because I don't want to run out.)
If you're wanting four fabrics in roughly equal amounts, you need two yards each. (rounding up) Eight fabrics, one yard each. Twelve fabrics, 3/4 yd each.
Hope that makes sense. :) Have fun with it!