This what you mean?
http://www.zend.com/manual/function.session-cache-expire.php
<?php
/* set the cache limiter to 'private' */
session_cache_limiter('private');
$cache_limiter = session_cache_limiter();
/* set the cache expire to 30 minutes */
session_cache_expire (30);
$cache_expire = session_cache_expire();
/* start the session */
session_start();
echo "The cache limiter is now set to $cache_limiter</ br>";
echo "The cached session pages expire after $cache_expire minutes";
?>
