The "rule" is after every 8-10 hours of sewing, advice from a sewing machine dealer, or at least every project, but I change my needle when I notice it isn't stitching quite right, or it starts to skip stitches. I do, however, change the needle when I start to machine quilt, because by then I've spent a lot of time sewing the quilt top or one or two more and it needs to be changed.