Código PHP:
session.cookie_lifetime integer
session.cookie_lifetime specifies the lifetime of the cookie in seconds which is sent to the browser. The value 0 means "until the browser is closed." Defaults to 0. See also session_get_cookie_params() and session_set_cookie_params().
Note:
The expiration timestamp is set relative to the server time, which is not necessarily the same as the time in the clients browser.
Es decir, que si no estableces un tiempo de vida, la cookie vive hasta que cierres el navegador. El tiempo de expiración se obtiene del server.
Espero que sirva, saludos.