[masklinn]

Two of them at least are fairly short: a run-through of Limbo takes about 3-4h tops, and you can go through Bastion in under 10 (although you'll need quite a bit more time to do everything and get all the achievements).

I've yet to finish the other 3 so I don't know how long they last.

[DoublePlusWill]

Amnesia takes around 15 hours or so. Limbo only took me a mere 2.5 hours. Both Limbo and Bastion are very easy to play in short bursts though.