How long is the game? It’s in my backlog, and I play games from shortest to longest.
The main game takes between 6-8 hours on average to get the the true ending. If you don't meet the requirements for the true end the game will offer to speed run you back through the required options you missed after seeing the other endings which unlock automatically, so no need to play though the entire game multiple times.
To unlock all content you will need to find all they hidden eyes (one per chapter) which can take you a longer, but there is a handy list with hints in the extras menu (which unlocks after completing the prologue,) and a chapter select which unlocks after getting the true end. So it might be another 30 minutes to an hour to 100% and see everything the game has to offer. Also it is possible (though unlikely on a first play through) to unlock everything in a single play through.
Hope that helps!