It's never easy to estimate the length of a game, but if Ubisoft's word is anything to go by, then the upcoming Assassin's Creed 3 will contain over 30 hours of gameplay.
The studio says that the game will contain around 2.5 hours of non-interactive cinematics in addition to the 30 hours of gameplay it'll offer. Ubisoft seems fairly confident with the amount of content the game offers, having mentioned it in the first episode of the recently released "Inside Assassin's Creed 3" video. That's news, especially if you haven't found the time to watch it.
With that said, there's a good chance that the campaign itself will run a third of the game's total length, with optional sidequests, exploration and other bits of content making up the rest of the game—which would not be unlike the game's predecessors, including Revelations and Brotherhood.
The length of the game should be unsurprising given the wealth of content in all the previous games in the Assassin's Creed series, including the first ever title. Hopefully, players will spend much of their time on actual content instead of galavanting aimlessly about the wilderness.
Assassin's Creed 3 will be out on October 30 for the PS3 and Xbox 360. Those on the PC will have to wait about a month later to see the release of a port. The game is also planned for release on the Wii U, tentatively as an early launch title on the Nintendo console.