gistfile1.cs

var chatMessages = GetObservableOfChatMessages();
var chatIsIdle = chatMessages. _dontknow_ (TimeSpan.FromSeconds(5));

chatMessages.Subscribe (msg => Console.WriteLine("{0} says: {1}",msg.User,msg.Text));

//I want this to occur every time the chatmessages observable have been silent for 5 sec
//not just once
chatIsIdle.Subscribe ( ... => Console.WriteLine("Chat is idle...");

You could make an extension method, BufferUntilInactive

public static IObservable<IList<T>> BufferUntilInactive<T>(this IObservable<T> stream, TimeSpan delay)
{
    var closes = stream.Throttle(delay);
    return stream.Window(() => closes).SelectMany(window => window.ToList());
}

And just ignore the return values, so your above code looks like

var chatIsIdle = chatMessages.BufferUntilInactive(TimeSpan.FromSeconds(5));
chatIsIdle.Subscribe ( _ => Console.WriteLine("Chat is idle...");

I got this from a similar question I asked on SO a while back, so credit to Colonel Panic for his answer, http://stackoverflow.com/q/8849810/255231