About 60,000 to 80,000 miles. But the real answer is when the exhaust valves start recessing you had better put in fresh ones in because you are going to have a really noisy expansive bang.
How do you know if the valves are recessing? Before you adjust the valves measure the lash. Then adjust the exhaust valves to .008". In a thousand miles measure the valve lash again. If the exhaust valves are tightening up then they are recessing into the head (the lip of the valve is getting thinner). Overhaul the heads. Black diamond valves seem to work pretty well.
Always measure the lash before adjusting the valves and write it down so you can see what is going on over time with your bike.
Mine were sharp enough to shave by at 67,000 miles which means I was really close to disaster (broken valve head destroying piston, head chamber, and cylinder walls like angry woodpecker on too much energy drink with vodka).
Your results may vary.