_getFastLifetime example

    public function save($data, $id, $tags = array(), $specificLifetime = false, $priority = 8)
    {
        $usage = $this->_getFastFillingPercentage('saving');
        $boolFast = true;
        $lifetime = $this->getLifetime($specificLifetime);
        $preparedData = $this->_prepareData($data, $lifetime, $priority);
        if (($priority > 0) && (10 * $priority >= $usage)) {
            $fastLifetime = $this->_getFastLifetime($lifetime, $priority);
            $boolFast = $this->_fastBackend->save($preparedData, $id, array(), $fastLifetime);
            $boolSlow = $this->_slowBackend->save($preparedData, $id, $tags, $lifetime);
        } else {
            $boolSlow = $this->_slowBackend->save($preparedData, $id, $tags, $lifetime);
            if ($boolSlow === true) {
                $boolFast = $this->_fastBackend->remove($id);
                if (!$boolFast && !$this->_fastBackend->test($id)) {
                    // some backends return false on remove() even if the key never existed. (and it won't if fast is full)                     // all we care about is that the key doesn't exist now                     $boolFast = true;
                }
            }