Observable.defer(() -> {
  AtomicLong lastFinishTime = new AtomicLong(0);
  return Observable
    .interval(5, TimeUnit.SECONDS)
    .map(n -> System.currentTimeMillis())
    .doOnNext(t -> {
      if (t > lastFinishTime.get()) {
          task();
          lastFinishTime.set(System.currentTimeMillis());
      });
}