I think it's around 15 hours.
Edit: Rise is more like 45 with the DLC. I wouldn't bother with the 2013 DLC unless it's nearly free. I think it's one tomb and some outfits.
With TR 2013, or any games that have lots of side stuff and collectibles to scavenge via exploring, it really depends how much time you're willing to spend.
HowLongToBeat.com has it at 11.5 hrs main story, 15.5 hrs main + extra, and 20.5 hrs completionist.
Rise and Shadow it has as almost equal to one another at roughly 13 hrs, 22.5 hrs, and 33.5 hrs. To me, Shadow feels longer on a lot of collectibles because some are a pain to find. I never fully completed that one, but fully completed Rise at least twice. I would also say Shadow is a bit harder to play with everything turned to Hard, because it can be very hard to see navigational paths in places.
My least favorite of this trilogy is Shadow. Besides the convoluted exploration, which can often take you out of what little story immersion there is, it's exacerbated by often having to wear specific outfits in specific places. There was also often times when searching for collectibles, I found I had gone in from the wrong direction, and wound up having to fast travel to a completely different camp and start over. There are also places where I'd previously solved a puzzle., and some of it remained solved, but a certain missing element to unblock the path (since the object the puzzle's purpose was to destroy had respawned), was no longer there. They literally force you to find some things by going in only one way, and it got so annoying I gave up on collectibles because of it. Lastly, Shadow by far is the least optimized of the 3. There are places where you'll get a huge hitch at checkpoints on a 1080, but people on 1080 Tis didn't have the problem. A 1080 also yields plenty adequate FPS. I feel maybe they tried to cram too much into Shadow for the age of the engine used.