@silvers:
It's a manga guys… Luffy will die when Oda says he'll die. Speculation is useless.
Well unless Oda tells us a set date, I think we have a right to speculate even if it is useless. I, for one, find amusement in seeing what other people think and contributing my own thoughts. It never matters if it's relevant, just that it's something to think about.
Anyway, onto my thoughts.
While Gear 2nd doesn't shorten lifespan and while Gear 3rd has only been used in the manga (guessing) approximately five or six times since he learned it, I would have to say that the amount of life lost is not equivalent to the amount of times he's used it, but the grand total amount of time he has used it. One would think the longest time he's used Gear 3rd was his fight with Lucci, which I would estimate at about 10-15 minutes. The other times were usually for a one time attack before reverting, such as using it to destroy obstacles in his way, attacking Moria and the Pacifista (I think it was), and attacking guards in Impel Down. So at best, the estimated grand total of time using Gear 3rd would be about 30 minutes worth. If this were to be calculated by any means, I would probably say about a single month for every minute.
So, by my own calculations, the constant use of Gear 3rd would have set him back 30 months, or two and a half years. Also noting that, during the fight with Moria that he was also using Gear Second, I would multiply that time by about five. Estimating that he only used Gear Second and Third together for about 5 minutes, the end result would be tacking on an additional twenty months off, meaning a grand total of 50, being just under four and a half years.
Now, addressing Ivankov's claim of 10 years off of his life to work his antidote as well as 1010011010's theory about how the life span being reduced because Luffy managed to recover significantly quicker, I would say that seventeen hours had passed out of the Forty-Eight that he required. That would mean approximately 2 years and five months off of his life.
Also, all of this is approximation, speculation, and general theory. All of the facts that are present are common knowledge and everything written above is just estimation.
I think I'm a nerd, now. God, I hope not.