It all depends on what I'm working on. I had one quilt that took about a bobbin per block, and I'm using the M size bobbins - the big ones, with Bottom Line (a thin thread)! Here's a link to that quilt:
http://wheatridgestudios.blogspot.co...t-is-done.html
Everything in the background of the white blocks was pebbled to raise the design in the middle.