If you're playing just the main story in New Woodbury (church dungeon and tunnels dungeon) and you reach the credits, about an hour, maybe an hour and a half. If you're exploring, checking out the different towns (Otnemarcas, Carrasco), hunting treasures, trying out the alpha version of the tournament, and side quests, then maybe three to four hours. I've got a lot of stuff tucked away in this demo for exploration.
No matter if you just want the story experience or if you wanna explore the worlds built up so far, I just hope you have fun :)