A lot of the packs that you pay for have four quests, a folk and a costume that is unlocked. Imagine how long it takes to do one of the standard quests, it's about the same.
I'd say you get about an hour out of each pack if you didn't get stuck. Then there is often more time spent releasing the karma of the folk you just got. I bought all the packs and don't regret it.
The clue is in some of the names of the packs, like "The Origin of Belgae" pack, which explains some of Belgae's past. I typed the descriptions up for the
Downloadable Content here. Some of the packs are just for fun.
Maybe im just dull but I got about 3-4 hours of play out of the undersea pack (pack 2). I did finish the pack completely though i.e. releasing all of the rare folk's karma and finishing the quests to the highest standard (message from under the sea quest does not need to be finished fully in order to complete it).
I thought there were some tricky puzzles in this pack and I had to do a few quests a couple of times.