Some people say 8 hours. Some people say with each big project. I usually go with when the thread starts to shred. Or when I have been sewing on tough things like Velcro and Vinyl. Or when I am starting a design on something that is more than just a piece of fabric (like on a t-shirt or a towel).
Thanks for the advice. My needle has had more than 8 hours on it but it still seems to pierce the material just fine. Just dont want it to start messing up in the middle of a design. What a hassle!