When I first set up /reader I was keeping a rolling three months of posts. Looking back I don't really know why.
Once items are read that's it. I don't have an option to save items within the system and that's by design – if I want to keep track of a post I should be using Like or Bookmark as intended, adding labels where appropriate.
I have decided to only keep one month worth of items, automatically removing those older than 'now minus one month' unless it has not yet been marked as read. For one thing, it will keep the database size down.
