I just finished it then - but the game-clock last I looked was at around 50 hours. This seems to be more than what most people played it, but I really enjoyed it and feel like I could have explored a lot more. I played this game ALOT over the last two weeks - at least a couple of hours everyday. It didn't really click with me until about 10-15 hours in, and that has nothing to do with the story, it's just at that point the world really opens up and you get access to a lot of the higher end equipment which allows you to do some of the cooler stuff. I also didn't want it to end so I made sure I did basically everything I could, save for the collectibles, before I took on the last mission.
If it has videogame parents it's Tomb Raider and Witcher 3 - I would say it's a must have for a PS4 owner.
Storywise it's actually really good - but for at least the first half you think it's all world-design and no story, but then pieces start to fall into place and the whole world's history becomes really compelling.
It's not without it's faults, but it ranks highly as a PS4 game for me - I enjoyed it more than Uncharted 4.
EDIT: I'll also say this for my experience. I feel the game is designed to hook you in with the gameplay mechanics in the first out and slowly tease out all the things you can do. For the second half what keeps you going is the main questline reveals. It probably didn't play like this for everyone, but as soon as the best equipment appeared at the merchants I made sure I could afford it, and as soon as I could clear all the Longnecks and Cauldrons I did that.
By the end of the game you pretty much have a set strategy for dealing with the bigger (huger) machines which involve using different stages of ammo and attacks - that probably is the game's biggest downside: that you can formulate a basically unstoppable strategy for anything with your arsenal, as long as you have enough health and resources. That's probably fair enough if you've maxed out everything, though, I guess.