In an interview at Paxeast 2020 it was hinted that a 10-15h play through was expected on average for the main story. This is on par with most modern RPG titles. I'd be more interested in the quality of the story, rather then the developers padding it out for longer play times. However it would be nice to get an estimate on the 100% completions play through, and if they will have achievements / side quests and goals for us to aim for after the main story ?