Currently the automatic purge is randomly called 6% of the time. see core/ArchiveProcessing/Day.php line 91. Instead this script should be automatically called once per day for each archive table.
Records of whether it was called or not should be kept in piwik_option table, eg. "last_auto_purge_archive_blob_2008_01 = timestamp".
(In ) - fixes #577 Aautomatic purge should be automatically called once a day on every archive table