Did it 10.000km ago. My bike started consuming oil at 85.000km I switched over to the cheapest oil I could find. At 130.000km it consumed 1L every 1500km. I decided to change the rings and valve seals which did the trick. Cylinder looked nice and piston was still in spec (rings were out a bit). I grinded (?) the valves again. It costed me around 150euro and about 20hours of wrenging.