I try and change with every 8 to 10 hours. I also change needles for various jobs, embroidery, piecing, quilting. It really does make a difference if you use new,job specific needles. Think about it, a needle is really the cheapest part of sewing, compared to buying a sewing machine, fabric, thread. And yet, people will keep the same needle in forever and then wonder why their machine is just not working right. A sewing machine repair person once told me that it's amazing. People will come into his store because the machine is not working, he changes the needle and the machine works fine. As sewers we really need to change the needle more frequently.