As others said, there's very little fabric wasted with a OBW, but there can be a lot of fabric in the seam allowances. In the first OBW book, Maxine Rosenthal says to allow 9 yards for a "queen size or larger" quilt using hexagon blocks, 11.5 yards for octagon blocks. I wouldn't trust that estimate, since it doesn't define what size the hexagons are or what size the quilt will be, but it is a place to start.