See http://bytes.com/topic/php/insights/889606-setting-timeout-php-sessions

/***
* Starts a session with a specific timeout and a specific GC probability.
* @param int $timeout The number of seconds until it should time out.
* @param int $probability The probablity, in int percentage, that the garbage
* collection routine will be triggered right now.
* @param strint $cookie_domain The domain path for the cookie.
*/
function session_start_timeout($timeout=5, $probability=100, $cookie_domain=’/’) {
// Set the max lifetime
ini_set(“session.gc_maxlifetime”, $timeout);

// Set the session cookie to timout
ini_set(“session.cookie_lifetime”, $timeout);

// Change the save path. Sessions stored in teh same path
// all share the same lifetime; the lowest lifetime will be
// used for all. Therefore, for this to work, the session
// must be stored in a directory where only sessions sharing
// it’s lifetime are. Best to just dynamically create on.
$seperator = strstr(strtoupper(substr(PHP_OS, 0, 3)), “WIN”) ? “\\” : “/”;
$path = ini_get(“session.save_path”) . $seperator . “session_” . $timeout . “sec”;
if(!file_exists($path)) {
if(!mkdir($path, 600)) {
trigger_error(“Failed to create session save path directory ‘$path’. Check permissions.”, E_USER_ERROR);
}
}
ini_set(“session.save_path”, $path);

// Set the chance to trigger the garbage collection.
ini_set(“session.gc_probability”, $probability);
ini_set(“session.gc_divisor”, 100); // Should always be 100

// Start the session!
session_start();

// Renew the time left until this session times out.
// If you skip this, the session will time out based
// on the time when it was created, rather than when
// it was last used.
if(isset($_COOKIE[session_name()])) {
setcookie(session_name(), $_COOKIE[session_name()], time() + $timeout, $cookie_domain);
}
}