Hi!
My tip is following - step by step:

(1)
Put the expire time into app.yml :

all:

  project:

    my_expire_ttl: 3600 # this means 1 hour

(2)
On first visit put one session on server - for example at login.
In login action , you set the expire timestamp , "global":

  

if( ! $this->getUser()->hasAttribute('my_expire')  OR  $this->getUser()->getAttribute('my_expire') < time() )

{

	$this->getUser()->setAttribute('my_expire', sfConfig::get('app_project_my_expire_ttl') + time() );

}

(3)
You make an action, only for check the TTL expiring , later for ajax calls:

public function executeCheckMyExpire(sfWebRequest $request )

{



	$need_redirect = "";

	if( 

		$request->isXmlHttpRequest() 

			AND 

		( 

			! $this->getUser()->isAuthenticated() 

				OR

			$this->getUser()->getAttribute('my_expire') < time()

		)

	)

	{

		$need_redirect = "YOUR LINK TO REDIRECT IN AJAX RESPONSE";

	}

	

	return $this->renderText($need_redirect);

}

(4)
Finally in the template :

<script>

var only_one_in_one_time_flag = true;



function checkExpire()

{

	// only run the function in one thread!

	if(only_one_in_one_time_flag)

	{

		only_one_in_one_time_flag = false;

		jQuery.get("__MODULE__/checkMyExpire", function (data) {

			data = jQuery.trim(data);

			

			if(data.length > 10)

			{

				// you know, redirect need, cause there is a string

				window.location.href = data;

			}else

			{

				// do nothing, cause  the user is already logged in, or the expire flag is still alive.

			}

			// if the async call is ended, then function is callable again

			only_one_in_one_time_flag = true;

		});

		

	}

}



setInterval("checkExpire()",10000); // in 10 sec in enougth to call

</script>

******-
Thats it, you can make this with cookies, or other tricks too, but this is the most simple - i think.

I hope, i could help you!
Best regards :
Gergely Szilagyi