Depends, take a look at the rest of your settings in the Archiving page.

You might have set “Never delete Unread Articles”.

Also, that purge job is on a cron. I’m not sure how often it’ll run. Could be once a week or even month. Thousands of rss articles and links are only a few megabytes big. But you can push the “purge now” button at the bottom of the archiving page to check your settings.