SRV.7.5 Session Timeouts In the HTTP protocol, there is no explicit termination signal when a client is no longer active. This means that the only mechanism that can be used to indicate when a client is no longer active is a timeout period. The default timeout period for sessions is defined by the servlet container and can be obtained via the getMaxInactiveInterval method of the HttpSession interface. This timeout can be changed by the Developer using the setMaxInactiveInterval method of the HttpSession interface. The timeout periods used by these methods are defined in seconds. By definition, if the timeout period for a session is set to -1, the session will never expire.
and from the DTD,
<!-- The session-timeout element defines the default session timeout interval for all sessions created in this web application. The specified timeout must be expressed in a whole number of minutes. If the timeout is 0 or less, the container ensures the default behaviour of sessions is never to time out.
Used in: session-config --> <!ELEMENT session-timeout (#PCDATA)>