To have Gnus automagically archive your mail according to it’s date, you can add the following to your .gnus:

  (defun my-fancy-expiry (group)
    "Returns the folder to expire to, as <group>.Archive.YYYY.MM, where
  YYYY is the year, and MM is the month (in case it isn't obvious).
  When using nnml, it's not a good idea to have the name <group>.YYYY as
  this may conflict with the message numbers (now, suppose you have more
  than 2000 messages in your folder?), that's why there's Archive in
  This is currently hardcoded for nnml."
    (let* ((target (concat "nnml: class="string">"  group ".Archive.%Y.%m"))
  	  `(("from" ".*" ,target))))
      (nnmail-fancy-expiry-target group)))
  (setq nnmail-expiry-target 'my-fancy-expiry)

You can have mail you read automatically marked as expirable by setting gnus-auto-expirable-newsgroups to a regexp that matches some mail group names. If you’d like this matching to be case (in)sensitive, but don’t want to fool with the global value of case-fold-search, you can do it by advising the function gnus-group-auto-expirable-p.

  (setq my/gnus-auto-expirable-case-fold-search nil)
  (defadvice gnus-group-auto-expirable-p 
    (around my/gnus-auto-expirable-case-fold-search (name) activate)
    (let ((case-fold-search my/gnus-auto-expirable-case-fold-search))