I extracted cooldown data from the CTC100 USB around 1pm today (~50 hours of cooldown). I've attached a log plot below. The heater RTD seemed to be behaving weirdly at the beginning, but soon stabilized and cooled as expected.

I estimated the time constant for the workpiece: 50 hr/ (300K - 90K) = 0.24 hr/K =  ~860 s/K