It's considerably longer. Like somewhere between 2-3x longer.

Also, RoTR is less linear and not as tightly paced than TR 2013. In addition, the majority of your time is spent doing semi-freeform exploration rather than shooting or being corridorded down set pieces (shootouts are both sparse and relatively short). You often go up to an hour or more without killing anything but wildlife in many parts of the game.

By "semi-freeform" I mean you have a distant goal marker telling you the spot you need to reach, but how you get up/around to it is up to you (though it often is fairly straightforward) -- but it's freeform in the sense that on the way there you'll happen upon loads of (typically non-combat oriented) distactions, such as optional tombs (mechanical puzzles), collectables just out of reach (navigation puzzles), mission-giving NPCs etc;

The downside is that despite having a more diverse range of combat options combat simply doesn't control as tightly or feel as satisfying as TR 2013. For example bullet sponge enemies cluster in waves, encouraging area-of-effect bomb spam; little annoyances like the item pickup button being on the same button as your reload (many times you'll reload instead of picking up a much needed bomb) and erratic auto-aim behaviour (how much it assists and when it snaps your aim directly to an enemy is seemingly decided at random).

But like I said, combat makes up a considerably smaller portion of this overall bigger Tomb Raider game.