Musically, I don’t think they’ll ever run out of ideas. JP alone mentioned how he’s always coming up with seed ideas, so the potential for writing new music most likely won’t be the reason they throw in the towel someday. All the guys in the band come across as having the drive and creativity to continue on until their bodies just give up. They all seem to be in great shape, with probably no heavy drugs or or heavy boozing involved as far as I know, so they’ll probably last past what average statistics suggest as long as no external disaster occurs.
From past interviews I’ve watched and read, all their families are behind them 100%, so I doubt they’ll ever get pressure from them along the lines of “Honey, I think it’s time to settle down.” Their families seem 100% behind them, which is an important factor (not too sure about Manginis situation, though. But I’m betting based on the guys personality, dedication, and acceptance of the other guys that he’s in a similar boat as the others).
As far as a farewell tour, I can only see it happening under a couple conditions. That would be either based on them collectively coming to the conclusion that they wouldn’t have enough physical steam to go beyond one more tour, or one of them eventually has a sit down with the others and says they can’t physically continue for much longer. JP himself said shortly after hiring MM that the current lineup was it, and they’d be done if they had to consider changing up again.
To conclude based on that, I think we have quite a few years left and plenty of albums to come that’ll just extend their already diverse catalogue!