At 415C (google AI's temperature advice) it was giving off no steam at all. It didn't start liberating bound water until 550C at which it was so much, that I didn't even need a mirror. I could actually SEE the steam. And by 680C - where I held it for 15 minutes, it petered out and stopped.
But I want to point out that if I'd believed Google's AI, I would have used the mirror, saw no steam, assumed all the water had already been boiled off, and considered the colemanite calcined. 5/