I had just watched all the cinematics on youtube (would never buy an XBO to play this but am interested in the story) and yup, they totaled to about 45 min. It was such a tease too... because the dude's video was 1.5 hrs long... he played through them twice for some reason... I was starting to think "damn, this game feels LONG" lol. them bam, it ends. Totally a Halo 2 style ending, that's for sure.
So 15 missions across 4.75 hours... that's about 19 min per mission, which is pretty perfect average mission length, IMO... Honestly I'd be happier with an average closer to 15 min.
Edit: huh... watching your review, now, so 3 of those 15 missions can be completed in under
two