I'll suspect there are rather a lot of owners who will never check them, myself included (84,000 miles up and not checked). Most mechanics will tell you that these 5 valve Yamaha heads will stay in service for the life of most of the bikes.
I read that there's some women in the States who had her first check at something over 200,000 miles and just one valve was on the cusp of needing a reshim. Impressive.