they just had one not too long ago (couple of weeks or so...I want to recall)
However, I think that was a "targeted" sale to frequent travelers via email (not open to general public)
so I think you will have to wait a bit for another flash sale to happen. Though it looks like Hilton likes to do the flash sale where you have 72hours to make a booking (under the sale) so they might be more inclined to run this sort of sale more frequently
we will keep our eyes open
follow many frequent traveler blogs/sites and they are all pretty quick to jump on any sale taking place