New Apple Pencil for iPad Pro 9.7" - Replace Tips When?
My new iPad Pro 9.7" should be here tomorrow. How often do iPad Pro users change the tips on the Apple Pencil? What sort of wear should I be looking for?
Use Apple Pencil with iPad Pro - Apple Support
http://www.macworld.co.uk/feature/ipad/10-tips-tricks-for-apple-pencil-3631354/
No one exactly says. i would say when it starts to look worn or you see significant differences in appearance.