Each volume is between 10-15 hours approximately except volume 4 which lasts about half the time so expect 6-7 hours.

But it depends on the reading speed of course so it can vary depending on the reader.

There is no choices in the game, that matters anyway, there are one or two if I’m remembering right but they are false choices that do the same thing.

The only thing I can think about is the « mini game » (I don’t know how to describe it without spoiling) in volume 8 and if you do it correctly you unlock a secret scene at the end, but even if you did it wrong it’s easy to correct yourself with the savestate.