The Trash and the Spam folders in your Gmail have a unique auto-purge property and old email messages belonging to either of these labels are automatically deleted after a fixed period of time (30 days to be precise). →
Get rid of unwanted and older email messages with auto-purge in Gmail.
For instance, if you have created a rule in Gmail that archives all your newsletter subscriptions to a particular label, these unwanted messages will live forever until you manually purge them. With auto-purging enabled, the older mails of a label are automatically removed from your mailbox.