EVERY 8 HOURS OF SEWING TIME!!! or more often depending on the projects you are working on. sooner is good longer...not so good. some dealers will tell you to change your needle with every new project as a good guideline. and you should clean your machine BETWEEN EVERY BOBBIN! every time you run out of bobbin thread it is time to stop and clean the machine again. your machine will love you for a long long time if you maintain it properly.