I have made several OBW quilts. There are so many variables it is really hard to say how much fabric you would need. It depends on the size of triangles you cut, the pattern repeat etc. If it was me I would probably buy enough fabric to make 2 sets. 12 repeats of the design. Unless the repeat is pretty short. The repeat on the last one I made was 18 inches. Then you can adjust your quilt size by adding borders. (does this make sense?) Good luck. I look forward to seeing it. I have one on my design wall right now. I just can't decide on my layout for it. I have changed it 4 times so far. I am usually not this indecisive.