Roughly eight million Frenchmen served in WW1 and just over one million were KIA. So roughly 1/8 died. This is a horrifically high casualty rate compared to most other modern conflicts, but on par for WW1 (most western front combatants were in that 1/6 to 1/10 range).

I'm not sure how you would put a 'lifespan' on it. There are way too many variables like time, location, and branch of service, and the fact that even with historically high casualty rates the vast majority of them survived.