I personally think the moment she "falls" is a crucial moment, but there is a little bit of time for an after story that is still hot and exciting. It highly depends how that "falling to the antagonist" is portrayed of course.
Is it a "FMC POV" scene where the player gets to see it but not the MC (yet)?
Is it a scene that shows to the MC that the FMC has "fallen" (he observes her having sex or she tells him in some way)?
Some games have phases where she has fallen and the MC has interactions with her that hint towards that. Like her cancelling plans to be with the bull. Or her being upset and her and the bull leaving for the evening just for her to come back the next morning being super happy and blushy. Or her not pushing back on the bull's rude comments when she previously has regularly done so and if asked about it kind of excusing it and pretending its not a big deal. Or maybe the bull being brisk and making comments alluding to her potentially being a good wife or marrying him or some kind of half-sexist half-flirtatious comment and she reacts very embarrassed and swooned and tries to play it off.
I think those kinds of interactions are really good for a post-fallen state or a state where the FMC is kind of in the process or very close to "falling" to him for good.
I do think however that these kinds of scenes can overstay their welcome, that you can prolong it too much and then it looses appeal and can potentially even hurt the story overall.
This is just me pulling bs numbers out of my ass, but I think the prologue / pre-NTR intro should probably take up anything between 5-20% of the story. Prologue/Intro should probably be relatively short with 5%-ish.
Then you got the phase where we further establish and characterize relationships between MC, FMC, bull and other characters - but where some scenes already happen. Arguments (i.e. FMC in conflict with bull establishing kind of a dislike/hate kind of relationship), sexual harassment, embarrassing moments for MC/FMC or sexually charged moments, moments that allude to sexual preferences of the FMC maybe (being into dominant men for example, without maybe knowing herself or wanting to accept that), why the bull is into her / is starting to target her (i.e. she is very abrasive and doesnt take shit at first and giving him a lot of push back and calling him out and he is into that). That is another 10-15% (ish).
After that you got the main meat of the story (we are at ~ 15-20% now) where the bull targets FMC and anything from circumstantial moments over quick boundary pushing over coercion, blackmail or even seduction attempts happens. This is normally the part of the story where we gradually go from sexually more ambivalent scenes like harassment, comments, groping and (coerced) "dates" or spending time with the bull over sexuall explicit (and often humiliating) services like the usual handjob, blowjob, footjob, clit-licking (getting her off), use of toys etc. over to definite things like 69, slight penetrations, service in presence of or near the MC up to penetration (first time and/or deflowering is a key moment ofc). I would say that takes up at least 40% of the story (we'd be at 60% now) up to 45% maybe.
After that is goes into extended corruption. Exhibitionism/public exposure, sexual experimentation, pet play, clothing changes, visible attitude changes, embarassing moments where FMC tries to hide what is happening, etc, etc. The focus here switches between the experiences of the MC and his POV (mostly (but not entirely) clueless is my preference, I am just going from that). It's important to highlight the MCs POV experience here cause its exciting to discover all these little signs that the MC sometimes notices but often doesn't. I think that should be around 15-20% (now we are at 75-85%).
The other 10-15% could be post-falling moment. FMC neglects MC more and more. More obvious appearance and attitude changes, thoughts about future life (marriage, etc.). More dreamy "into-bull" FMC, more open signs of affection of the bull towards FMC that are not being rejected anymore and just very flimsily excused. Now the MC definitely notices, but does ofc not have all the background knowlege on how long it went on and just thinks that maybe FMC fell "normally" to the bull/antagonist. And then the seperation from MC is probably the highlight with a marriage or smth official. There can be a timeskip or some other form of "post-credits" section basically. Japanese games like to call that the "AFTER" or "APPENDIX". There can be a lot of variations ofc. Like in a fantasy setting the adventuring party splits off and FMC travels with bull. Or you all continue traveling and the NTR goes on while the MC is pretty sure now or even knows it (kind of going more into the depressing helpless acceptance direction).
But yea. That is kind of how I would build it in my head.
