From a quick check of the patreon, Chapter 1 had six updates and Chapter 2 had sixteen. From everything we've seen so far of its development, I think it is safe to assume that Chapter 3 will be of a similar length to Chapter 2, if not even longer. Other than that, we have no idea and likely won't be able to guess until we're well into the double digits of Update 10 or 11 at the earliest.
With the speed at which the story is currently going and with the amount of focus on the extra characters and the environment (also the number of extra characters is insane), i just don't see how this game could be finished in 16-18 Updates. I know it's futile to even mention numbers this big but to me, 14 to 16 would be the ideal number of scenes each of the 4 girls should be getting (especially when the first few are as mild as the ones we got so far in CH3), after all they're the reason people play the game in the first place.
16 Updates worked for CH2 for many reasons but however i look at it it's just not viable for CH3, for example there was the village in CH2, it consisted of 8 or maybe 10 houses and everyone who lived there was either an antagonist, a guy getting cucked or a girl being corrupted meaning that everything that the residents of the village did, in some shape of form, progressed the story, there were no NPC's that need to be worked on constantly, like changing their everyday habits and writing new dialogue for them every once in a while like what we have now in CH3 with guards, vendors, sailors at the docks or even the random people at the tavern, and the extra characters were not only fewer in numbers but they had 1 or 2 scenes maximum (Alys already has 2 just in the Recollection room), not to mention the pacing was WAY faster, Solana and Thania started off with a blowjob and Neve's second scene was anal, while in CH3 we're about to get the third update later this month and we got nothing so far with the 4 main girls.
Don't get me wrong these are not bad things whatsoever, the pacing is great for the long run and the city feels MUCH more alive, but these improvements shouldn't come at the expense of the girls' corruption, i'm not saying they will, but trying to squeeze the game into a box that's the same size or slightly bigger as the one it's predecessor was in, with this many new stuff added to the game, is not doable without taking something away. If the scenes for extra characters are gonna stay this frequent, the number of scenes per Update stays 3, and there will be this much focus on keeping the world "alive", 17-18 Updates won't be enough to properly see the girls' corruption through. Take Rose for example, she made an appearance in Update 1, then she got benched in Update 2, now she's not gonna make the final cut for Update 3, and she's probably coming back in Update 4.
Even though it might not happen, i still hope that by the time we get to the end, all 4 girls will have arrows here pointing to the next room because the first ones are already filled out (if my math adds up 12 scenes fit in a room, which means it takes 16 Updates with 3scene/Update just to fill out these 4 rooms). I know that half of the lewd stuff that'll happen in game won't qualify as a proper "scene" so it's not gonna make it here (for example the freeroam events), therefore the game could still be fulfilling even if some of the Recollection rooms look half empty after the game is finished, but i still believe that filling these rooms takes precedence. I might be the odd one out but when i first found out about CH2 (around Update 11) and saw all those provided saves, i loaded the most recent one up, watched every scene in the Recollection room and was done with the game, i didn't even play it through until the first ending was added.