This setting defines the number of days since a junk comment or TrackBack was received or marked as Junk until the item is deleted.

The value of this setting depends completely on how often you look in your Junk folder for false positives. The default is 60 days, but most people will be satisfied with 7 days.

Please note: There is a known issue in Movable Type 3.2 related to this:

The auto-expiration of Junk feedback items only occurs when a user visits the main menu for that weblog. In future versions, auto-expiration of junk items will be triggered system-wide when a new junk feedback item is received.