When we do a flash sale, its ending time is dictated by a purchase that triggers one of the BAM! Big Bonuses.  When a purchase triggers one, the number remaining is reduced by one. Once all are spoken for, the flash sale is over.  You can see how many are remaining by looking at the left pane on the Purchase page at the bottom.  When that number hits 0, the sale will no longer generate bonus perks and reverts back to normal.  Be sure to double check what the current number is by refreshing the page before you purchase if you've had the page up for a while.