The main story of Call of Duty: Modern Warfare 3 lasts around five hours, according to HowLongToBeat. However, you have to be solely focused on the story or just really good to beat it in that time. If you go for some of the achievements as well, then it will take around seven hours.
When focusing on the main objectives, Call of Duty: Modern Warfare 3 is about 6 Hours in length. If you're a gamer that strives to see all aspects of the game, you are likely to spend around 40 Hours to obtain 100% completion.