alot of people change to a new needle for each new project, also, at least every 8 hours of sewing time but i have found if i am doing some really heavy quilting i may need to change my needle every 4-6 hours. it also depends on the fabrics/batting being quilted and the thread being used. good rule of thumb would be between each project and as soon as you notice any change in stitch quality; if your thread starts to break or stitches start to skip it could be time.