Googling “Assassin’s Creed II too short” will return you around 176,000 entries. However, players who actually have completed Assassin's Creed II are in the minority according to Ubisoft's Gaelec Simard.
The Assassin's Creed: Brotherhood mission director told Joystiq that only 40 percent of players finished the game.
The stat was arrived at from tracking by Ubisoft’s Uplay online service. Simard says the figure is pretty decent, (guess)timating 5-10 percent as a typical completion standard.
”We want the player to experience the whole package, so that's something we're trying to push,” he said.
Anyone worried about the length-rate of Brotherhood, however, shouldn't be: Simard told VG247 at gamescom the story will last 15 hours.