With Assassin's Creed: Revelations concluding Ezio's extended quest (and the "Embers" animated short providing a dignified epilogue), the plot's modern component -- starring feeble hoodie-hermit Desmond -- is set to run its course by the end of 2012, just in time for next year's AC outing. Creative lead Alexandre Amancio explains it to Eurogamer: "In Assassin's Creed we set up a timeline with this whole end of the world plot of December 2012. That's fast approaching, and the story we have to tell, we obviously need to do it before we arrive at that point."
Desmond's tale is far from being the final one in Assassin's Creed, but Amancio believes that having the current protagonist's dilemma linger after calendar 2013 would be "stupid," given the authors' efforts to run parallel to reality. "It would be stupid of us to be centring [sic] a game on a semi-reality and then have that conclusion happen after that date in real life," he says.
That's assuming we all survive beyond 2012 anyway.