@abstractmethod<EOL><INDENT>def get_idle_timeout(self, session_key):<DEDENT>
pass<EOL>
Returns the time that the associated session may remain idle before
expiring.

- A negative return value means the session will never expire.
- A non-negative return value (0 or greater) means the session
  expiration will occur if idle for that length of time.

raises InvalidSessionException if the session has been stopped or
expired prior to calling this method