Midgar was 4-5 hours of gameplay stretched to 40 odd. Most if not all of that expansion made sense, with the amount of characters being introduced and expanded on, areas that had huge potential to be grown (Wall Market, Sector 7, etc etc) and a bit of poetic licence with new content. Leaving the city was an obvious point to stop Part 1 because where in the next 4-5 hours of the OG would you end it?
Anything longer than 40 hours in Part 1 would've been excessive to me, but with the amount of potential in the areas coming up and the slightly less linear nature of the story, why couldn't this be a 60-70 hour Part 2 that also ends with something very significant (e.g. Temple of the Ancients).
Assuming the game follows the major elements of the OG, we've still got an awful lot going on and plenty of areas to be expanded upon. In fact just about every area could have some detail added. Junon (it's massive), Gongaga (Zack), Corel (Barret's backstory), Gold Saucer (sidequest central), and so on.
So the question really is where is the significant moment that you can end Part 2 on? In my mind it's either the return to Nibelheim with the encounter in the mansion, Temple of the Ancients or even the Forgotten City. I appreciate that's a bit non-committal but after the way Chapter 18 played out it could literally be anything.
Anyway It'll all be thrown up in the air when that first trailer drops with a Godo or Cid reveal...!!!