Hi, and thank you for your interest in the game!
Currently, two out of three difficulty levels are available for the main story, the latter which is unlocked after beating the former and which can be played without having to start over. To beat just the first difficulty setting will take you anywhere between 5-10 hours depending on your party composition, luck with drops and overall proficiency at the game. Then you have three more optional quest types that will offer unique challenges and rewards, as well as a casino. Engaging with either of those would bring playtime up by another few hours.
I hope that could clear things up for you!