Dull needles actually poke holes in your fabric, so you don't want to keep needles for too long, or your quilt will not last as long. I change mine every project, or about 10-12 hours of sewing. You can "feel" your needle to see if it is dull, a new needle feels sharp, and does not have burrs on it. If it doesn't feel sharp, or you can feel bumps along the sides, it is time to replace it.