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.
It is not rude in any way to say you will be away. In fact, if you will be away for a long time, it's a good thing to do in a comment, not in your question. It is not a necessary part of a question, and it will probably be edited out. In a comment, however, it will be kept and read.