By the time we hit the credits of Echoes of Wisdom, our playtime was just above 20 hours. This does include collecting most of the game's Heart Pieces, and Accessories, and completing a large number of Side Quests, as well as taking multiple screenshots.
If you just focus on the Main Quest, we estimate Echoes of Wisdom's length is around 15-17 hours. You can probably sneak in a few Heart Pieces too in that time.