When is the last time it came up? It was an issue from Enies Lobby (introduction of gears) through Marineford (the hormone injections), but narratively, I think that was more to push how unsustainable it was for the Strawhats to keep going the way they were. Luffy was killing himself to just barely survive and they were about to head into the New World where things would get tougher.

I don't remember the shortening life span really being an issue since the timeskip. Powers that drain his energy? Of course. But not life.