Actually it lasts forever or more precisely, an indefinite amount of time, because there is no reaction in any of Rossi's device. Anyway, none has been demonstrated properly. Ever. I suspect that Mills' reactions are similar. They last until a short time after he turns off the huge power input and the only reason that even happens if because of the thermal capacity of the apparatus.
That is an interesting proposal, that it is only the retained thermal energy of Mills SunCell apparatus which causes the plasma to persist for so long after the electricity is turned off. I don't think it stands up to scrutiny however. In fact I have yet to hear any plausible mainstream scenario why hydrogen gas and trace amounts of water, in a high current, low voltage environment, should produce such an energetic plasma at all, let alone one that is more energetic than the input energy.