TakeUntilIdleFor

rx.cs

public static class Ex {
        public static IObservable<T> TakeUntilIdleFor<T>(this IObservable<T> source, TimeSpan idleTime)
        {
            return Observable.Create<T>(o =>
            {
                var published = source.Publish();
                var idle = published.Select(_ => Observable.Timer(idleTime)).Switch();
                return new CompositeDisposable(published.Connect(), published.TakeUntil(idle).Subscribe(o));
            });
        }
}