RequestAdditionalTime has no effect

Question

I have a Windows Service that Imports files into DB. When the Service receives to stop notification, It should complete the current Import if possible. It can take several minutes to complete.
Therefore I use the ServiceBase.RequestAdditionalTime method to Signal the SCM that the Service is still working and operable.

protected override void OnStop()
{
   var waitTime = TimeSpan.FromSeconds(5);
   var additionalWaitTime = TimeSpan.FromMinutes(3);

   Trace.Write("Stopping service...");

   var task = Task.Factory.StartNew(() => worker.Stop());
   while (!task.Wait(waitTime))
   {
     Trace.Write("Requesting additional time...");

     RequestAdditionalTime((int)additionalWaitTime.TotalMilliseconds);

     Trace.Write("Waiting for {1} to complete...");
   }

   Trace.Write("Service stopped.");
}

While testing, I cannot find any different behaviour of the Service using the RequestAdditionalTime method. If I remove the RequestAdditionalTime call, the Service behaves the same way:

• On Service Stop, the SCM waits for about 2 minutes and reports the Service is not responding (Error 1053). After that, the Service state remains Stopping until my worker completes.
• On Shutdown, the System does not wait for the additional time. It seems to kill all Services after 5 seconds (Registry Key WaitToKillServiceTimeout)

Question

What effect does the RequestAdditionalTime have? Where am I supposed to see a difference when using it and how can i test it? Several times i read that the SCM will kill the Service if no additional time requested. But I couldn't see that behaviour.

All tests where run on my local development machine (Win 8.1), assuming that the behaviour is the same as it will be on a Windows Server OS.