I felt like, main story-wise, it's gonna last (typical visual novels) 6 chapters, but what about side stories? Only the schizo/other Fen (or Fen's possible relative, depends on how you speculate) had been revealed to be involved in one of the side stories, what about the others? And how long will they last?

I can't say how long exactly it will be but you can see the plan for what's to come here in this public post: https://www.patreon.com/posts/flaming-flagon-100566644
It should give you a better scope of the project-- though of course there are obviously some spoilers of upcoming stuff so read as you will.