So my stronghold has started to give me opportunities for minor adventures, but I'm not sure how I should use them.  I assume the experience goes to the idle character that undertakes the adventure, so it seems like it's pointless to use a character unless I'm going to use that character again.  I hired a ranger and a cleric early on, but I've replaced them with the pre-written NPC's who are now higher level than the ones I hired.

 

So should I dismiss one of my active party members so I can send them on this adventure to take advantage of the experience gained?  How long is a turn anyway?

Almost everything about the stronghold is pointless, so do whatever. Don't send your active companions on adventures, though. Turns depend on done quests so you will have to play with an incomplete team, which is counterproductive. Your active companions gain XP as you play, no need to get rid of them for minor XP bonuses.