@redthumbCorrect, with a few wrinkles. The unarchiving is (or at least initially was) not automated, so there may be some delay there. An archived author may have some stories non-premier, where these were written together with another author who has not yet had their work archived (and, if that author then becomes eligible for archiving, such stories go behind the paywall immediately without the week's grace/notice). On the authorlist pages, archived authors are noted by a check rather than solid background. The last posting date on those pages is the last posting of story content OR blog entries, and for some authors this posting could have been done by site management so will appear to be too recent. Management posts for an archived author (which can occur if the author agreed to posting of content sourced from elsewhere) will be behind the paywall immediately (they may be publicly available for a few hours, until the next archive check). An author's works are archived when the author has not logged on to the site (even to read stories or check mail) for some while; initially this period was five years, but it is currently being slowly shrunk down to three years. A corollary is that all pseudonyms of an author are archived together, although note not all authors archived together are pseudonyms. Archive checks/actions are run at 5:30 GMT/BST AM&PM, except a day or so around the clocks changing where the checks may occur an hour earlier/later so nobody has to come in on a Sunday to make the change, and so that the check has been performed by the correct time. It's impossible to tell if a story was already premier-only when the author is archived; presumably, such stories will remain premier-only if the author is unarchived. On the authorlist pages, if an author has an email link, then they have logged on to the site within the past year, even if they've not posted anything for over a decade - the converse is not necessarily true, as an author may elect not to show an email link.
This info is derived mainly from direct observation and from Lazeez's postings and notices, but isn't official as I'm just a site user.