Other than a handful of hints offered by Ubisoft and the knowledge that Ezio will no longer be the main character of the series, we know very little about Assassin's Creed 3. Everything from Japan to America has been named as a potential setting, and time settings have been even more dicey with people guessing all sorts of things.
CVG reckons all that's about to change, thanks to an unnamed source who says the next game will be announced before the end of March. The source also says that the setting will be North America and that the game will take place during the American Revolution. That means you can expect appearances from George Washington, Thomas Jefferson and other important figures of the time. It's also worth noting that George Washington already has a role in the Assassin's Creed fiction, as a holder of an Apple of Eden.
If it's true that Assassin's Creed won't be taking a break this year, you can expect the next game to be released around November.