The campaign takes 6-7 hours if that. I was actually surprised by how quickly it was over. It looks amazingly pretty, but that campaign is extremely short in the modern market. I'm actually genuinely disappointed in it as a result and just hope I can get my money's worth out of the multiplayer instead.
@GryphonICD Did you play it on Easy? The thing with Halo campaigns is unless you play on Legendary, it's far too easy (except Halo 2. Fuck Legendary on Halo 2.) It took me about 12 hours on Legendary, and while I never felt overwhelmed to the point where I'd think "I'm never gonna beat this," it was challenging the entire way through, and I enjoyed it immensely.