If it's turn based I think it can be a very long game. Perhaps 300 - 400 hours? Basically turn based combat are very slow. With that I think they can jack up the play time tremendously in my opinion.
PoE2 was RTwP. But even for that game was long enough for me. Maybe was it the biggest difficulty?
Whatever the system Larian is using I really hope they make the combat super addictive. Even you are already a skilled player, game can still be challenging. For example DOTA2.