I saw posted in another thread the following:

Empty (but not completed) instances stay around for 5 minutes before they close.
Can you change the time to one minute or less? I find that when I'm looking for a group to join I have to scroll through a lot of empty dungeons before I find one that I want to join. Honestly, I'm trying to think of a reason to keep them around at all once the last player leaves, but I can't think of one. I would be all for removing them immediately once the last player leaves, unless someone can explain why that would be a bad idea.

As always, thanks to the devs for this great game!