Andy's been playing playing Halo 5 at Gamescom and has some interesting takes on the new style of gameplay - faster action and more tradition guns, for example. One thing that stood out though was mention that Halo 5's campaign is twice as long as Halo 4, a game that wasn't exactly an afternoon's diversion. 

You can find out more in the video which I lifted from Andy's feature. According to what he was told at Gamescom it's supposed to be twice as long as the campaign in Halo 4 (which he points out was "was quite a long campaign"). He was also told that things actually started bigger and were cut back. The suspected reason for the length, he feels, is partly due to the game spending time in all three of the Human, Covenant and Forerunner worlds. Watch the video for that and some other facts.

https://www.youtube.com/watch?v=OiFjmFCAAr8

http://www.gamesradar.com/halo-5-apparently-twice-size-halo-4-s-campaign-and-it-started-bigger/

Sooooo ten hours?

roughly 16 hours if this is correct

http://howlongtobeat.com/game.php?id=4265

Length was never the problem. I'd take an 8 hour campaign as long as the level design, story, and AI is improved.

I wonder how many chapters does it have, Halo 2 has the most with 14/15 if I'm not mistaken. So H5 will have, what, like 25/26?

Halo 2 was the longest Halo campaign at least for me. I can't imagine a Halo campaign being that long though, looking forward to seeing how long it really is. 2 more months to go!

Well there's so much to cover with Blue team doing their stuff, Locke's team hunting Chief. The stuff unfolding in Hunt the Truth. Guardians, Didact, Jul 'Mdama, the UEG. I expect this to have the deepest story of any Halo game.