I am guilty of not changing my needles often enough, although when the stitches go wonky or don't look nice any more, I'll change the needle. I was told by the man who sold me my Janome that I should change the needle after every project, and/or after 8 hours of sewing. First, I can only do small pieces of time sewing because of shoulder issues, so I'd lose track of the hours, but when the machine goes wonky, I will change the needle and then I'll wonder why I didn't do it sooner.