<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_expiration_time(self, app: 'Quart', session: SessionMixin) -> Optional[datetime]: """Helper method to return the Session expiration time. If the session is not 'permanent' it will expire as and when the browser stops accessing the app. """

        if session.permanent:
            return datetime.utcnow() + app.permanent_session_lifetime
        else:
            return None