Just finished!
I just fishished this game and it took me:

Actual story: about 1~2 week
Whole thing: about 3~4 weeks?
I'm a slow player :P

Anyway the review.
You don't need much Japanese knowledge to play it because you can test anything out.
I know that it's better because the story + dialogues made me burst in to tears :_:
and you can read the rewards, etc.
Fantastic graphic, music and gameplay. The characters are totally cuuutttteee! (especially Pukurin/ Wigllytuff in English?)

Well, that's all I can say at the moment. A great game for all ages especially Pokemon fans and who enjoyed the past dungeon game.