setting and extending Session Lifetime using Zend_Auth

i use Zend_Auth for one of my Projects, but so far haven't figured out how to set the Lifetime for the Session, or how to extend it (lets say it should run 5 minutes and should reset to that when the user makes an action), here is my Initialization code:

        $authAdapter = new Zend_Auth_Adapter_DbTable($this->_model->pdo);
        $authAdapter->setTableName('normal_folks')
           ->setIdentityColumn('username')
           ->setCredentialColumn('password');

        $post = $this->_request->getPost();

        $authAdapter->setIdentity($post['username'])
            ->setCredential($post['password']);
        $auth = Zend_Auth::getInstance();
        $result = $auth->authenticate($authAdapter);

        if($result->isValid())
        {
            $userInfo = $authAdapter->getResultRowObject(null, 'password');
            $authStorage = $auth->getStorage();
            $authStorage->write($userInfo);

            if(strlen($post['refferer']) > 1){
                header("Location: ".$post['refferer']);
            }elseif(strlen($this->_request->getParam('ref_action')) > 1){
                Zend_Controller_Action::_forward($this->_request->getParam('ref_action'),"admin",null,null);
            }else{
                Zend_Controller_Action::_forward("index","admin",null,null);
            }
        }

Ant this how i check if the user is logged in:

                if(Zend_Auth::getInstance()->hasIdentity()){
                    echo "Woho!";
                }else{
                    die("invalid-identity");
                }

Its probably right there in front of me but I just can't figure it out, help? Please? Pretty Please? :D