It's 15. Looking at the newest wave of Campaign previews, some sites say the Locke canyony mission, titled [spoiler]Swords of Sanghelios[/spoiler], is two thirds of the way into the Campaign.
Other sites have said it's the tenth level. So my meagre arithmetic abilities tell me that puts H5 level with H2, ignoring intro cinematics and training 'level' Armoury, at 15 playable levels.
So, another H2 split? 8 levels with John and 7 with ... Wait. Was H2 13 playable levels?
H5 bigger than H2's space opera confirmed.
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At a guess, I'm going to say the average level is going to be at least a little longer than Halo 4. Just based off the engagement zones I've seen and the general set up. I'm hoping for a long campaign. They've played off the dynamic of it very well up until now.