A new interview with Toshiyuki Kusakihara and Genki Yokota with French video game site JeuxVideo has revealed that it takes roughly 80 hours to complete a storyline for each house in the upcoming Fire Emblem: Three Houses. Here’s a quote from the translated interview courtesy of Serenesforest.
Kasukihara: To complete the game with just one of the three houses, it took me 80 hours. I did not skip the dialogue or cutscenes, of course. So if you want to do the three paths, it can take you more than 200 hours.