Depending on the projects I have been sewing, I probably most of the time change my needle about every 10-12 hours of sewing...that really is a lot of sewing...and if there are any signs of problems I will change it sooner...such as skipped stitches, any noises such as a thump, or any other problems with stitching. Quilting through layers of fabric will dull a needle faster than piecing, and sewing through fabrics other than cottons also will dull the needles faster. Really shouldn't wait until you break a needle to change it.