When are questions and answers archived and what is the criteria for deciding whether to archive them or not.
Questions and answers are always available to the public and never archived from view, so everything is always accessible.
If you want an idea of the size of the content, you can take a look at the archived dump. There's actually far more information that is not shared in this way.
If by "archived" you meant "deleting the questions so that space on the server is preserved", then the answer is never.
Stack Exchange in general only does soft-deletion; the content is almost never purged from the server/database (i.e. hard-deletion). Deleted posts are only hidden to some users, not necessarily deleted from the server/database. Any users with 10k+ rep and above have an ability to see those deleted posts.
However, some questions are indeed automatically deleted when they meet some criteria (though unrelated to preserving space). The process is called roomba.