Well, they said that the first part is a full fledged game. With that I expect 40+ hours, though could be bigger. Anything less isn't remotely a full FF game. I think my completion time on them range between 70-100 hours (grinding included).
That said, still no word from them as to how the continuation will be. Will episode 2 start right as you leave the city? Can you grind to level 99 in the first part, and start off as that in the next part? Which would be drastically different than the base game. Or will it be like the Trails games where you only transfer a few items and choices made in the game and start the next game at a slightly higher level.
Have high hopes for the first episode as FF7 is one of my all time favourite games, but I don't like the split episodes at all. 2022 for next part and 2024-25 for last? That might even be generous. And that is if they keep it at three like originally stated, which I think they backtracked on. E3 2029 - Coming this winter, PS6! Launch title and exclusive, final part of FF 7.
Did notice though how the intro from the demo didn't have any mention of being an episodic title. Just said remake.