A while back I wrote a post about a fellow named Aaron who rode his KTM 500 EXC around New Zealand then shipped it to South America and rode from there to Alaska. During that trip Aaron put more than 35,000+ kilometers on the motor without any major work. He changes the oil and filter regularly and had the valves checked in San Diego (they were in spec).
Aaron is still riding. After visiting Alaska, Aaron headed south to the mountains of Colorado and is currently enjoying the scenery and riding in Moab. He has ridden the 500 EXC more than 60,000 kilometer and has just over 800 hours on the motor, still with the original top end. He did recently replace the clutch plates though.
You can read his on-going ride report here.
Here is his bike in Moab.