Ninjas are very tough and durable if properly cared for. The most most mileage I've heard of is over 80K miles( a bike now owned by LSR champion and Ninjette forum member Racer X).
I have over 38K miles on my streamlined pre-gen. Alan Smith has 50K on his streamliner. Both bikes are still running strong and very competitive in the Vetter fuel economy challenges.
