Mew provides a mechanism to guess a folder where the parent message of a current message was refiled before.
For example, pooh, piglet, and roo had a chat to go and get honey. So, pooh made +project/honey then refiled the message to it. The further messages, if they are properly replied, they are supposed to be refiled to +project/honey.
Information that which folder was chosen for messages is stored to "~/Mail/.mew-refile-msgid-alist". `mew-lisp-max-length' controls the amount of this information. The default value is 1000 messages. If you want limits it to 2000 messages, put the following to "~/.emacs".
(setq mew-lisp-max-length 2000)
The function name to provide this feature is `mew-refile-guess-by-message-id'.