Gears of War 3's single player campaign will at the minimum 10 hours, but most gameplay runthroughs will top 12 hours.
According to CliffyB, "I can tell you this is the longest and thickest campaign Epic has ever produced. Gears 1 and 2 were roughly about eight or nine hours for the average person, it's at least a few hours more than that. It's thick - and it's not just copy and paste level design. There's all this unique combat, all these story elements; there's a lot of stuff going on. It's quite a journey."
Gears of War 3 is due to be released September 20, 2011, along with a newly Gears of War 3 Limited Edition bundle including two controllers and a 320 GB specially modded Xbox 360.