How long can a fight like this one continue though? For Tobi, everything from #636 onward until #655 was dedicated to him. It included several chapters worth of despair and rebound cycles, some paternal bonding, and the climactic TnJ at the end. Unless Kishi chooses to recycle some of those topics for upcoming chapters, the focus will be centred on the Alliance's struggle against Madara. For the time being, there doesn't appear to be any sliver of uncertainty or remorse in his heart unlike Tobi, which means that everyone will be giving it everything they have to defeat/kill him.
I'm not sure how long Kishi could possibly drag this fight out without the Shinju returning as an enemy to divide the attention of the Alliance forces and Black Zetsu in Tobi's body can provide only so much content (this will likely be his death battle). These final confrontations of the war could last for only a volume's worth; assuming it pans out like the fight between Naruto, Sasuke, Tobirama, and Minato against Tobi. It may depend on how far Kishi is willing to go with the expository speeches to prolong the story (chapter #646 being the most recent example of this).
Many fans are already arguing that the ending of the manga will arrive by summer of 2014. That's only about roughly 20-28 chapters worth of content. Personally, I think that's unlikely. The Naruto and Sasuke fight from the original series almost reached that amount by itself. You can rest assured that their rematch at the end will at least be as long as that, if not longer. No... I think the manga will continue until the end of 2014 at least. This fight against Madara and the aftermath will probably consume all of the content going into Spring. Then there is still Orochimaru, Taka, and others to deal with.