All email messages are automatically deleted after 30 days. There is no need to manually delete messages but if you prefer to clear out your quarantine, you can do so one page at a time by clicking on the "Delete Spam" button at the bottom of each page:
If you need to recover a message that was accidentally deleted then you can do so by using the "Include Deleted Spam" checkbox at the top of the page.
For those that are interested in controlling how long messages are retained, consider voting on this feature request, here:
(This link will prompt you to login)