I'd like to say I change after each project, but since I'm working on 3-4 things at a time that wouldn't work for me. I usually have piecing to do, applique to be sewn, and quilting to be done. Once I finish with the quilting and binding of a quilt (cuz I'm too lazy to do it by hand) I take my bobbin case out, clean the whole machine and change the needle so I can start with a "clean slate".