Even with everything going for it, I guess the absolute limit will be determined by the lifespan of Rosetta.
From the June 23rd "Kamikaze" article: "By October 2016, the probe's propellant will run out and the retreat from the Sun will make its solar panels less useful. So the ESA team will make a series of orbital maneuvers to bring the probe down for a hard and terminal landing on the comet's surface."
That said, perhaps Philae could be put back into sleep mode to keep it alive until the comet swings back around our way again. In which case, who knows? A future rendezvous with another probe would be pretty cool, albeit a tad impractical.
Biting the hand that feeds IT
