If singleplayer is still a big part of why you enjoy playing FPS games, then it's hard to justify spending £50/$60 for six hours. The campaigns in shooters like the original Homefront, Battlefield 1 and Titanfall 2 range from 4.5 to six hours in length. For the most part, other developers followed its lead. Modern Warfare 2 took 6.5, and the series has lingered there ever since. According to one of my favourite utility sites, How Long To Beat, COD 2 took an average of 8.5 hours to beat.
When did five or six hours become the target length for a shooter campaign? I think it was during the journey from Call of Duty 2 to Modern Warfare and then Modern Warfare 2.
