Honda provides a very useful and accurate service interval guide in the factory service manual. It's very useful for answering these questions.
In spite of what people might desperately want to believe, these engine start losing ring seal and along with it horsepower around the 5-7 hour mark. It's a single compression ring piston design with very little skirt to support the ring package. As a direct result the rings wear very quickly and start leaking badly.
All that fantastic high rpm power your engine made new comes at a cost in the long run. It's not an XR so unfortunately you can't treat it like one at maintenance time.
I am not sure how the piston/ring design differs with the YZFs but my sons YZ450F had a complete topend done about 15 hours run time ago. This weekend I noticed a strong air push coming out of a vent at the head and a small amount of oil dripping. Concerned I called Rich and he informed me that is a normal indicator of the rings beginning to need replacement and posibly also the piston. Granted this was 15 (20 at most) very hard high rev hours but it puts into perspective these machines even when serviced by pros (Rich and Eric Gorr in this case) need constant maintenance.