If you have a premium subscription items should remain unread for 30 days (from their publish date). Nothing to configure.
I think there has also been a limit on the number of items saved for a feed since I’ve seen items go unread before 30 days in some high volume feeds. Don’t think that limit exist anymore since the archive tier is a thing now, either way, for this to be an issue in a matter of days there would have to be hundreds of new items every day in the feed.