Hm~ From what I've ever seen, I would say sometime in the Fall or Winter months. Usually it's twice a year, sometimes three though! ^^ I was kind of disappointing with the summer/spring ones, the Petri Dishes... But, eh... It's okay! ^^ But new ones should be out in a few months~!
Oh! And if you want some Microbes for Christmas, I suggest ordering them a month or so in advance, they can get a TON of orders and yeah~!