I just beat the campaign for Halo 5. It took me 11 hours and 52 minutes. I beat the first 2 and a half missions on legendary with a friend. I then finished the rest of the game solo on heroic. Yes, I spent some time looking for intel (75/117), and I only found 1 skull. Anyone that beat the game in less than 5 hours must have played on easy or normal without looking for any collectibles, and literally rushed through the game. If I played solo all the way through on heroic , it would have taken probably around 9-10 hours.
Btw, the campaign wasn't bad, I enjoyed it, but not as good as previous Halo's.