Considering that game mag reviewers almost never finish the RPGs they review (at least, not before the reviews are written), they probably pulled those numbers from two locations:
The upper end of 80 is probably something provided by the publisher as a typical playtime and is probably much longer than an actual typical playthrough (gamelengths.com lists the average playtime at 52 hours)
The lower end of 12 most likely comes from the fact that in order to get the most powerful weapon in the game, you have to reach the end of the final dungeon in under 12 hours. Sort of a speed-run side quest. So again, not a typical playtime.
On behalf of all the people who have downloaded this without saying thanks or clicking "like", and on behalf of all the people in the future who will download this without saying thanks or clicking "like," I'd like to say, thanks! (and I clicked "like.")
For anyone out there who has ever thought about being considerate and showing their gratitude but was afraid it would take up too much of their time, I would like to say that I accomplished all of this in less than a minute. Imagine that. How many minutes do you suppose it took to scan and edit this magazine for your benefit, hmm?