gnus-total-expirable-newsgroups

gnus-total-expirable-newsgroups is a variable defined in `gnus.el'.
Its value is

nil

Documentation:
*Groups in which to perform expiry of all read articles.
Use with extreme caution. All groups that match this regexp will be
expiring - which means that all read articles will be deleted after
(say) one week. (This only goes for mail groups and the like, of
course.)

You can customize this variable.