Shelter (about an hour-and-a-half)

Neighbors from Hell: around 5~6 hrs., unless you want to achieve 100% then it could double its duration.

I guess some parts could have been challenging requiring several retries, but I don't recall it being even near impossible or anything. And it certainly took me a full day to finish it, not just a couple of hours. A better game might be able to finish it in a few hours, maybe.

Overall I thought it was an ok platform jumping game with a dark atmosphere.

Portal is another game for me which I finished "in a day", as far as i recall, but it is not on GOG of course. Never got bored during that time, just kept playing and playing until I finished it. i really liked the last boss fight, one of the funniest and most humorous gaming experiences in my life.

Yes. Portal is a fun little gem.

More from Oh Sir:

these are some of mine:
Your son poses nude for the communists are tells dirty jokes at funerals!
Your sense of style is vile and your son can't tie a tie!
Your favorite Bond actor was defeated by a frightened schoolboy, and that's a load of bollocks!
Your son died because of you and is no more, mister! (this to God)

the ai used these on me, they were very effective for some reason:
You were born in your country!
Your father has a steaming romp with your mother!

Thank you very much for all your suggestions. I've already played Hexagon, Oh Sir!!!, FTL, Shelter, Neighbours from Hell, Brothers, Limbo, Gomo, Chronology and the Shivah (which I think is the shortest)... Nevertheless, I will settle with Mainlining, Tower of Guns or Akalabeth. Thank you once more for your help!

i say 140, and you can beat blackwell bundle (point&click) games fairly quickly, faster if you know the answers of the puzzles...

Journey of a Roach
Short, fun...
I completed it in about 2.5 hours... one can probably play it in a lot less time.