Writing unit test for a method containing RxJava's throttleLast operator

I've got a method that I'd like to test that looks something like this:

 private Disposable getStuff(){
    return Observable
            .combineLatest(
                    repoOne.getStuff(),
                    repoTwo.getOtherStuff(),
                    session.getSomethingElse(),
                    session.getOtherSomethingElse(),
                    (a, b, c, d) -> new Stuff.Builder()
                            .stuff(a)
                            .stuffB(b)
                            .stuffC(c)
                            .stuffD(d)
                            .build())
            .throttleLast(500, TimeUnit.MILLISECONDS)
            .subscribeOn(mSchedulerProvider.io())
            .observeOn(mSchedulerProvider.ui())
            .subscribe(this::populateStuff, this::showError);
}

I'm trying to test this method but it doesn't get passed throttleLast. I've tried it mostly by applying a TestScheduler, but didn't get anywhere with it. For example I've tried:

<...>
    .throttleLast(500, TimeUnit.MILLISECONDS,mTestScheduler)
<...>

Then when I call the method, I advance the time:

    mCompositeDisposable.add(getStuff());
    mTestScheduler.advanceTimeTo(0, TimeUnit.MILLISECONDS);
    mTestScheduler.advanceTimeTo(505, TimeUnit.MILLISECONDS);

But that does nothing, i.e. populateStuff or showError is still not called. I'm probably using the TestScheduler incorrectly, any suggestions?