Ubisoft told VG247 at Gamescom that single player campaign of Assassin's Creed: Brotherhood would take about 15 hours to complete. It's unknown whether this includes side missions, but as always these kind of estimates should be considered an estimation at best.
With the quick turnaround between Assassin's Creed II and Assassin's Creed: Brotherhood, there has been fears that the campaign would be very short this time around. But seemingly these fears can now be put to rest. Assassin's Creed: Brotherhood will also be the first game in the series to feature a multiplayer mode.