How to limit submissions "1 submission every month"

Thanks, #2 worked for me, I set it once per year

$interval_options = array(
  '-1' => t('ever'),
  '60' => t('every minute'),
  '300' => t('every 5 minutes'),
  '3600' => t('every hour'),
  '86400' => t('every day'),
  '604800' => t('every week'),
  '31536000' => t('every year'), // Added this line for "Every Year"
);

However i believe per year should be added in the default options