How long will Luffy last?
Currently luffy is only 19 yet he's already done a lot that he could be considered a legend.
He's gone up against numerous shicibukia, defeating some and befriending others, gone up against admirals and come out alive.
Punched a tenryubito, broken in and out of impel down, defeated CP9, led to destruction of Enies Lobby, survived the battle at Marine HQ. The list goes on. He's certainly done more than a lot of people can muster in numerous lifetimes.
However, he's sustained considerable damage from numerous opponents, he lost 10 years of his life when Ivankov healed him and Rob lucci said his gear second move is like doping and so is hacking at his lifespan (which he does so much now its second nature to him) . In addition to always pushing himself so much that he's knocked out and faints after most battles. Its certainly a lot for someone so young.
Just like Roger conquered the sea, becoming the Pirate king, he fell to a disease before turning himself in and getting executed.
I think luffy will follow a similar fate, he's just 19 yet he's steadily inching close to admiral/yonkou power level (not there yet). In exchange for all this, I doubt he'll live long.