I'm talking about the hard enrage of the boss in phase 2. Upon entering phase 2 of the fight in 16m hm you have roughly 8 minutes and 30 seconds to kill TFB. If the boss is not completely dead within that time, he will enrage, regardless of what part of the phase he is in or what tentacles are alive or dead.
Thanks for providing the estimate of hard enrage time. Will keep an eye on his hp/time when we tackle 16m.