I've got an active question, but I will be away until the new year. Is it OK to add a comment saying I'll be away or is it rude?
I always appreciate it when someone does this. I think that you should stick around for at least 20 minutes after posing a question or answer that might need clarifying. After that I would think it's really fine to walk away.
If you asked a really dramatic question or something that might need clarifying I would consider it rude if you immediately left a comment saying: "Ok I am leaving you guys to interpret that however you want. Won't be back for two weeks!" Barring that I wouldn't consider this rude in the slightest.
I always appreciate when someone overcommunicates in this way and lets other users know what is going on.