Checked mine at 25,000 miles (first time), dead in the center, don't plan on checking them again till 50,000. I run synthetic 5W-40 changed every 3,000 miles, 30 mile commute... year round rider at a typical 70mph for 20 of those miles, surface streets at 40-45 mph for the rest of the ride. Very seldom redline the bike maybe 5 times in 32,000 miles. Not sure if this has anything to do with it or not. Other than the valve adjustments I'm pretty anal about everything else. Every 2 year change all the fluids , air cleaner at 15,000 mile intervals, etc, etc.
06 DL650, 70,000+