I was hoping that the flowers on Hoya imperialis would open before the blooms faded on Hoya macgillivrayi and Hoya archboldiana and I was lucky enough that it did.  Here is a comparison photo of the three flowers held in my wife’s hands:

Here is a photo of the flowers in a shallow bowl of water.  These will sit on my desk until they fade away.  The large Hoya flowers usually last at least two days when kept this way.