Every year I adjust two or three valves, they are not off much, maybe .0005". They do not move a lot in the run of a year but I still like to know considering the access ease. I am sure I could go twice as long for the valve interval as they do not seem to regress into the seats and rocker wear seems non-existent. Did my YZFR6 around 40,000K and they were all good, now my Jetski needs them done every 100 hours with a couple of valves usually needing smaller shims due to marine conditions, but it's always at high power settings gobbling fuel.