Purging PROCESS and DISCARD folders. What is good criteria?

I'm looking in my PROCESS and DISCARD folders, and see messages from months and months ago.

What is a good overall policy to know when these messages can be purged? 

When WF moves a message to the PROCESS directory, does it ever try to use that message again?

When WF moves a message to the DISCARD directory, does it ever try to use that message again?

Seems like I can just create a rule "remove messages older than 3 months" from both of these folders directly on that email account.  Anyone do it like this?

Thank you,

John