This will vary depending on the model and style of the motorcycle and how hard it is run. Modern 4-stroke engines with titanium valves that are raced and ridden very hard will probably need to have the valves replaced and valve seats cut every 100 engine hours or so. If you cruise around the trails and are not spending a lot of time in the higher RPM ranges you will probably get quite a bit more time out of it. If you are racing and constantly bouncing off the rev limiter you may need to have these things done more often.