I believe off the top of my head that it was around 13 hours.
Well, the side missions get repetitive, as there are only a couple different types, but I quite enjoyed them, and I didn't think they overstayed their welcome. There were a couple missions that were extremely good, but everything was pretty consistent in terms of quality. I found the story to be quite good, and even liked the ending (I've seen a couple complaints so far).
I actually mentioned earlier that I could never get into the 2 previous games. I've played each of them through PS+, but I never cared for Cole or the setting. I want to go back and play through them now, because this game was amazing. I watched partial playthroughs of the previous games on YouTube in preparation for this one, and, as I mentioned earlier, the cutscenes, acting, writing, graphics, and just about everything else are on a completely different level. It is so much improved from inFamous 2. It truly is next-gen.