Hi there, The rule of thumb is that you change your needle with every new project. Don’t forget to make sure you’re using the correct needle with the right fabric. Do I always follow this rule? No, but don’t tell anyone. I usually change it every few projects and have never had any problems. But if you really want to ensure optimum sharpness of the needle, then you change it often. Definitely sign up for your local fabric stores sales flier so you can be notified when machine needles go on sale and then you can build up your stock so you’re always ready.