Aw, man. I hate to say it, but the thought of Naruto ending anytime soon or something really bums me out. Not that that has much to do with this thread, since it's asking about the Naruto versus Sasuke fight and when it will be, but still. We all know the ending of the series itself won't be long after it. As for when the fight will be...
I can't really say. Yeah, the War Arc in going pretty fast and smooth, but Kishi could always throw in something that actually makes the series go into, technically, a third part. I know people have been spreading the rumor of a third part to Naruto for a long time and it's probably just a silly rumor, but really. Naruto is one of those series that not many want to see end; who knows how much longer it'll go and just how much longer we have until the infamous Sasuke versus Naruto battle.
But, to answer the actual question, I'd guess somewhere between mid 2012 and early 2013.