I haven't played ME2 since I beat it earlier this year. I've been wanting to get the DLC, but I don't want to get them if they are too short.

Katsumi and Overlord will take you a few hours, but Shadowbroker is a good length. I recommend Katsumi and Shadowbroker if you have to be selective, Katsumi's quest is shorter but you can use her for the rest of the game, and Shadowbroker is just a fun adventure that ties off a big loose plot thread.

Kasumi is one fairly short mission, but it's different and nets you a new squadmate. Overlord is a grind and lasts about an hour or so.

I've only played Shadowbroker but that is like 3-5 hours depending on how much you die and how Completionist about it you are.  I really don't know if it would have felt worth my money if my main Shep weren't faithful to Liara throughout 2 (I don't think most DLC is any good though and this was at least worthwhile).

Overlord and Shadow Broker are about 3-4 hours each, and the Kasumi DLC is about an hour.

I loved all the dlc offerings for ME2 , so much that I wish that must of the missions in ME3 are of the same kind ( lenght /objectives / variety of gameplay) I would say go for it , specially Lair of the Shadow broker , also Kasumi is way cool to pass her up , operation is more of a stand alone akin to Bring down the sky from the first Me , I liked it but IDK if you will , each last about 2-3 hours more of less , depends on the difficulty really , and your skill.