No idea. We had one failure of a stock exhaust in 10 years of racing including Daytona and Road America and RA Superbike School on the same old used valves out of one of our junk motors. I have no idea how to extrapolate that to any one else's situation.
You could do the analysis and testing I suppose, but that would be more expensive than replacing valves every season. Ask K/W what they recommend for replacement times. They may not know, but it's worth asking. The answer might be 500 hours at less than say 11,000 and immediate replacement if motor went past 12000 or some such. Life depends on so many variables including revs, cycles, and temperature and let's not forget that excessive clearances make the valve slam back onto the seat and that shortens life dramatically too.
The real issue is not the cost of valve replacements or even piston, crank and barrel replacement, but how much is invested in the head. We ruined a CL175 sloper head and had to fit the one from my only other motor and that will be expensive to weld up and machine. Our heads are ported and flow tested and machined for different squish etc and that all takes time and other scarce resources.
The smart thing to do is to fit K/W valves on a racer and keep a good race log. All parts have a fatigue life and will fail eventually. The trick is knowing when and swapping them out before that point.