how long does the story & 100% achievements take?

[mib]

PLEASE NO SPOILERS! i've only just beaten the bit where you fight walnut and the rainbow bird flies away!

anyway, started it a few weeks ago and its super fun, and the non-hidden achievements look pretty reasonable (haven't looked at the hidden ones yet). how long to beat says it takes about 120 hours to beat everything, but i'm about 35hrs in and still in part 1 and my highlest level character is ash at level 42.

[Fel]

Chapter 1 (or part 1) sets the world and characters so it takes a decent amount of time, with the most episodes as well.

You are probably grinding a decent amount if you took that long to get to where you are, which is fine if it was by choice.
If it wasn't by choice then you might want to lookup how to level up a bit more efficiently because the remaining chapters (especially the last one) have much tougher enemies.

The hidden achievements are mostly tied to post-game stuff, which are pretty grindy (much more than the main story).

Keep in mind that this game can be beaten with a variety of different team compositions, ranging from Marona only (more of a challenge run but perfectly doable) to a whole group of combatants (usually 2-4 "main" combatants).
Grinding can be for a variety of reasons as well, and having a decent idea of what item you want to use for each character can make things quite a bit faster too.