Add an option for every 30 mins

Drupal cron has a minimal resolution of 1 hour. Providing a 30 minutes option in Notify will do nothing if you just rely on Drupal cron.

If you need more granularity than one hour you need to set up an external Cron or Elysia Cron to run as frequently as you want notifications sent. Then in Notify, change the setting for "Send notifications every" to "Cron".