Dragon's Lair Trilogy can be completed in one day. It's my first time playing them, but it took me 2 hours 6 minutes according to Galaxy to finish the 3 games included.
Blackwood Crossing was another game that can be completed in one sitting.

If its not mentioned before oxen free can be done in a day but what a game and can be played multiple times too. The telltale series games too

I've recentle finished the first Deponia in less than 9 hours (not played in the same day tho)

I have this thread bookmarked but I wonder if the list is till being supported at all or if it has been abandoned - since last edit was 2017 if I am not mistaken.

I'm monitoring this tread too.
Afaik Gragar left GOG a while ago, but that doesn't mean much around here.
In any case, you could write him and whether he replies or not, I'm pretty sure someone else could take over the list in a new thread.

Grargar left? Oh no - he was always so helpful with his knowledge. Why must everybody leave here?

It's always about GOG changing their policies, priorities and promises.