code: def get_expiration_time(self, app: 'Quart', session: SessionMixin) -> Optional[datetime]:
          
          if session.permanent:
              return datetime.utcnow() + app.permanent_session_lifetime
          else:
              return None
docs: 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.