Re: Bioshock Infinite
My playthrough took about 17-18 hours (going by my playtime on Steam). This was on the hard difficulty and I tried to explore as much as I could. It was a VERY thorough playthrough. I did miss a couple voxophones though.
Some of the later voxophones reveal some pretty big elements of the story. They're not essential to the gameplay in any way but the later ones especially you should try to get to understand the big picture.