public function deleteExpiredRequests()
    {
        $helper = Mage::helper('sheep_debug');
        if (!$helper->isEnabled()) {
            return 'skipped: module is disabled.';
        }

        if ($helper->getPersistLifetime() == 0) {
            return 'skipped: lifetime is set to 0';
        }

        $expirationDate = $this->getExpirationDate(date(self::DATE_FORMAT));
        $table = $this->getRequestsTable();
        $deleteSql = "DELETE FROM {$table} WHERE date <= '{$expirationDate}'";

        /** @var Magento_Db_Adapter_Pdo_Mysql $connection */
        $connection = Mage::getSingleton('core/resource')->getConnection('core_write');

        /** @var Varien_Db_Statement_Pdo_Mysql $result */
        $result = $connection->query($deleteSql);
        return "{$result->rowCount()} requests deleted";
    }