Just finished 100%ing it today on "Hard".
If I had to throw a ballpark figure, I'd say it took maybe 11 or so hours.
Some of those collectibles are annoying to find. Felt like I spent 2 hours just on the beach section looking for shit.
My opinion of the game has remained unchanged with this new version. It is OK. There's a thread already about what a sequel needs which nails the majority of my wants too and gripes with this game. But in short: needs puzzles, traps, better sense of vulnerability and isolation, harder platforming, short shorts, and dual pistols. The CoDification with respect to the levelling up, perks, attachments, combat, and optional collectibles doesn't bother me as much. Prefer the combat in this one to the other games. Doing backflips and sideflips with the pistols in the PS1 games was neither fun nor sensible. They need to keep the awesome traversal this game has. Rope arrows and climbing axe add to it. And finally, a better story and characters would be cool, but ultimately TR is all about gameplay for me so that part isn't as important.