C++(11) Timer: Accumulate until idle

I'm facing a rather basic problem that I'd like to implement in C++(11 is Ok) using only standard libs.

Assume a function "Message()" which can be called any given number of times, after it's not called for a given time I want to trigger an action. For example:

Message();
Message();
Message();
Message();
sleep(10); <-- the idle time triggers an action 
Message(); 

Hope this makes sense.

The way I implemented this so far is using a combination of an async while loop and a condition variable which I pulse every time a message comes in. Once the wait for the CV times out I take action.

In pseudo-code:

void Message(){
  unique_lock ul(M);
  new message = false;
  CV.notify();
}

...

future = std::async([]{ 
  do {
    unique_lock ul(M);
    new message = false;
    got_message_within_time = CV.wait_for(ul, 20ms, new_message==true);
  } while got_message_within_time;
  // Got a timeout here...
  time_to_take_action(); 
});

...

Message()
Message() 
Message()
sleep(10)

I'm not convinced this is the most elegant solution out there, anyone has better suggestions? All in all the fundamental statement I want to implement is: "once I stop calling you, do something"

Any help/suggestion is welcome

Thanks !