I change needles:
-When it looks like it's taking effort rather than being an effortless piercing of fabric
-after a large project if I think about it
-if the tip doesn't hurt my index finger at all when I press up on it
Janome takes Organ needles just fine (same manufacturer , I've heard). I use those, but I also found the titanium ones after I had a string of breakages. They really are impressive for performance and longevity (and I'm not easily impressed into spending more).