Resolution:

From AMX document -> Composite Development -> Bindings : SOAP Bindings.

  "Session Inactivity Timeout (s)": The length of time before an invocation of the endpoint times out. Default: 60.

A more detailed and understandable statement follows.

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This argument specifies the time in seconds between client requests before the servlet container will invalidate this session.

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A corresponding explanation is available in the javax.servlet.http.HttpSession API documentation.

  https://tomcat.apache.org/tomcat-5.5-doc/servletapi/javax/servlet/http/HttpSession.html#setMaxInactiveInterval(int)

The "Session Inactivity Timeout" is a value for the HTTP session, "javax.servlet.http.HttpSession" between client and server, used on the SOAP/HTTP Service Binding side. It is the time for which the Servlet Container (the SOAP/HTTP Server resides in the Servlet Container) will remain active between two calls or client accesses. After this time has elapsed, the Servlet will invalidate that particular session between the client and the server. 

By default, the session manager provided by the Jetty server sets an infinite inactive timeout value and may cause a potential session leak. In AMX, 60 seconds is the default inactive timeout and normally the user will not need to change it. Per the specification, a negative value for Session Inactivity Timeout indicates an infinite timeout, i.e., the session will be active for as long as the servlet container is alive. Typically, in a browser-based scenario, after the timeout has occurred the user will be redirected to a login page as happens with the AMX Admin.