Topics not replied to in over 30 days (or whatever I decide) will be archived. In other words, if a topic is active it will not be archived.
I personally think that archiving the topics which haven't been replied to in two months will be better. Otherwise you'll get casual users coming on and thinking that they've missed a lot last month because this month seems empty and start shifting through the archives. I don't think anyone would want to shift through any more than two months of topics anways.
So yeah, I vote those older than 2 months to be archived.