I’d be curious about burnout stats as well. But the concept itself seems to have only been coined very recently:

The term “burnout” was coined in the 1970s by the American psychologist Herbert Freudenberger.

So I guess it’ll be hard to come up with any stats.

In any case, I’d also suspect that in the past, burnout was generally less common.

I recently listened to some lectures on US history and one of the points they made is that one of the things the immigrants coming to the US in the 1800s had to adapt to was the expectation to work that much. Many immigrants came from cultures where leisure was much more common (IIRC, weddings were celebrated for several weeks in some places in Europe).