I reckon you could get through Cargo: The Quest for Gravity in a day!

Noticed my huge list of suggestions hasn't been added yet. D:

EDIT: To add to it, Uriel's Chasm takes about 30-45 minutes.

Earthworm Jim? Really? Not the first time you sit down with it, I think.

I'll add Octodad: Dadliest Catch and Jazzpunk from that list. Escape Goat is one I added myself a long while back. I am trying to focus on GOG games where possible, and also those that come within the 2-5 hour timeframe. There are an increasing number of "One-Hour Games" these days that only require an hour or less to finish (Shelter, Proteus, The Moon Sliver, several others) and am thinking these should have their own list. They are more like short-story experiences than games, at least in the sense of the word "game" that this list is for.

Added Eldritch. Took me about 4-4.5 hours to complete straight through, though there are extras and unlocked modes still to try. And I could replay and try for the "good end" outcome that I assume is there, since there are extra steps described to effect the best ending.

Haven't played them yet but I guess Vanishing Of Ethan Carter and Neverending Nightmare should be finishable in less than one day.

From what folks are posting that does sound like the case. I'll add them in once it's confirmed. Thanks!

Vanishing of Ethan Carter: 4 hours.
Neverending Nightmares: Between 1 and 2 hours, depending on if you go back for the alternate endings.

Thanks, sounds like they fit right in then. I'll get them on the list.

They Breathe can only be done in one sitting. I'm not even sure if checkpoints carry over if you close the game. But it's a fantastic experience all the same!

Thanks, added to the list. Another title I've had for a long time and not gotten around to. At least it can be completed in a day!

Many of Nifflas' Games can be done in one day like Knytt or Saira. The same applies for Daniel's Base games like Iji (which is already listed) or Hero Core. The free game Expendabros on Steam is also short (1-2h), but fun.

Risk of rain is also a game that can be done in 1-1.5h , but I guess it falls in the same category as FTL and Rogue Legacy so it won't get listed.

catpower1980 seems to disagree, at least with regards to Knytt Underground. :)

Yes, Risk Of Rain is more of a burst-play game that takes a number of rounds to truly complete, unless one is very good and very lucky the first few times. Two other games that I am currently playing which have short but incremental plays that improve your future condition are Luftrausers and Shattered Planet.

Hopefully for me, "many" doesn't count for "all" :o) Nifflas did so many games :
http://nifflas.ni2.se/

Concerning Knytt Underground the speedrun record is less than 30min but hey there are some über crazed gamers who finish Dark Souls in 1 hour :o)

Knytt Underground speedrun when you know all you have to do and where to go:
http://www.youtube.com/watch?v=mfm56y_psR4

What the completed map looks like considering that each little square is a complete screen filled with secrets/puzzles/enemies/platforming and you have to figure out how they connect to each other :o)
http://www.youtube.com/watch?v=eLX85pHAgbY
(endless walking/running edited out of the videos ^o^)

Thankfully one of the criteria for my list is "intended to be played in 2-5 hours", rather than speedrun. Similar to how navigation apps will give you an estimated walking time to your destination by assuming you are an average person rather than Usain Bolt. :)