I've actually been able to do one in about 6 hours. The "trick" was that it was my fourth (in a row), so I had the technique down cold. As you can imagine, boredom was my biggest challenge - but I had a deadline to meet!
I also cut my fabrics and batting at the very beginning and did almost no piecing of fabric and/or batting to make long-enough strips. This, of course, made it more expensive. To save time, I machine-sewed the binding on both sides. Using these techniques plus a serger, I've actually gotten the time down to 5 1/2 hours.
All that said, I greatly prefer taking my time and doing things right - especially attaching the second side of the binding by hand! I'm not proud of the quilts I've made in under 6 hours because anyone who knows anything about sewing/quilting can spot my shortcuts right off the bat.