Even with a medium density of FMQ quilting for a twin sized quilt, I very rarely go through 500 yards for the top....But I also generally also use a different thread for the bobbins depending on the color of my backing. For the bobbins, I often buy the large cones of thread and they seem to last forever! I try to buy them in colors I know I will use over and over again (dark greens/blues/tans, etc). I do more matching on the top colors.