What is the life expectancy of Ti valves?

Assuming we're not talking about those 'cup style 7MM (or what ever they're down to)stems, they last a long time. I just don't know on the very small stem valves. They may last as well. I'm sure heavy nitrous or other severe conditions would shorten their life.
We did a set of ex cup chevy heads way back in the 23° days. Used the same valves, and they saw 8 years in the super classes before I lost touch with them. No valve failures.
So, the material itself seems to be good for long life.

There are production vehicles with Ti valves, including the Corvette.

The valves are in a set of Indy 440-1, I am looking at buying. The vales are a couple seasons old. I was told that they might have a short life, and need to be replaced every few seasons. Just wanted some input as that would get expensive. the concern i have was that they would break do to fatigue. I don't want to add any reliefs on the tops of my piston this way(lol)