User blog comment:Argorrath/Activision confirms next Call of Duty for this year/@comment-6249853-20130209024214/@comment-5607850-20130209150743

That would mean it would take 4-6 years for one company to develope the game. You only need 2-4 years. The reason why there is a variation is because of the Treyarch, Sledgehammer, and IW each developing games.