How were you able to play LoG before its release?

He's either a dev/beta tester/reviewer who got an early copy

I am an editor for a German print magazine called PC Games.

18 hours sound like quite a good length.
I'm not that good at quickly getting through games, so I'll likely spend more time.

And if it was 10 hours for people who know the game and route to take, was it 2 hours the devs of Skyrim could complete the game in? But then again Skyrim is a rather nonlinear game. I guess there's less stuff to wander around and discover in Grimrock though.

I very liked the video review from Rock, Paper, Shotgun

http://youtu.be/Q1pRhK6J6K8

It took the reviewer 34 hours and he took his time with the secrets, puzzles and even skipped an area. He guess a focused player would need 18 hours.

Also bare in mind their is the option for new dungeons to be added latter on, either by the creator or the community. So if 15 odd hours isn't enough for you there can always be more.