On IGN’s live stream from Gamescom Josh Holmes just told Daemon Hatfield, “from the perspective of length, this is a campaign that you know if you’re familiar with Halo 4, this is about twice the length of that campaign.”
Gamescom 2015 Day 2 - IGN Live
You’ll have to search through the video (it’s around 1 hour 24 minutes into the stream) since the live stream won’t let me pull from specific point in the video. I also don’t know if they link will work after the stream is over, so I’ll have to update the post if it gets screwy.