I change my needles with every project or quilt. I have found that lots of times there is dust built up in the bobbin area that causes a 'thud' kind of sound but it's not always the needle that's making it. So I usually clean the bobbin area and then if I can still hear the 'thud' sound, I change the needle. I can, usually, piece a king sized quilt before I need a new needle. I only change my rotary cutting blade when it starts skipping places as I'm cutting.