How long does the top stay crackly after broiling the sugar. I need to take these to a dinner party where they will go in the fridge until dessert...

A better solution than broiling would be to pick up a small hand torch and and burn the tops in situ. You don't need the big daddy for this. You can find small butane fueled torches in better kitchen stores. That way you can do the job on the spot right before serving.