Final Fantasy XV is Half Open-World, Half Linear Progression
According to a new interview from Famitsu (as translated by the folk at Siliconera), Final Fantasy XVwill be neither entirely open-world or entirely linear. It will, instead, be a combination of both with open-world elements taking up the first half "because of the story".
Director Hajime Tabata stated that "if you do it like that, then you won't get tired of the open world, and as the whole game becomes more focuses, you can enjoy a traditionally Final Fantasy style progression".
Chapter-wise, Final Fantasy XV goes from zero to 15 and should last players between 40-50 hours.