Probably zilch since it's a Japanese-developed game with no UI, tutorials, or even text outside of a sign in the background in the opening of the game. The U.S. publisher (NIS America) sold physical copies on its online store--$19.99 for just the game, $29.99 for a Limited Edition that added an artbook, soundtrack CD, and "collector's box"--and that's the only way you could get one. Otherwise you have to go the digital route.
There's a review up at Destructoid:
http://www.destructoid.com/review-htol-niq-the-firefly-diary-287859.phtml
That score won't deter me from playing my copy once it arrives.