On the gamepage it says the Demo was a preview of the first 4 hours, it took me only 1 1/2 hours to finish. How long will the finished game be?
Sorry, slow reply here! It's very hard to measure gameplay time in an adventure game, especially a sizable and complex one like this. Some people will get stuck in places that others won't while some people will follow walkthroughs. Some people will skip through most of the dialog while some people will take the time to explore every optional puzzle and talk to characters about random STMs.
That being said, someone at GoG called this four hours based on their own first-time playthrough. I can play it in under an hour if I skip all the dialog!
There's a lot more game to come after this, but it's hard to say how long it will take each player. I can play through the entire game in about five hours by rushing through and skipping most of the dialog. On a normal playthrough, when I'm playing for enjoyment, listening to the voice acting, and covering most of the optional stuff, it takes me around 12 hours. If you're not me and you don't already know all the solutions to the puzzles, it should take you longer than that!
Hope that helps answer your question! Sorry I can't give you a specific number!