The population during the ~decade WWII covered was 2.3 billion, so about 23 billion person-years were lived, about 1% of the total.

You can argue that total down considerably -- other stuff happened during that decade, and while something like 10% of the population fought in the war, it wasn't truly global -- but it's still impressive on the scale of history. The US Revolutionary War was probably only 1/1000th of that.