This threshold is met the year we build a fusion reactor that has run continuously for at least 24 hours at engineering breakeven (Q_E >= 1).
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Engineering breakeven is defined here: https://en.wikipedia.org/wiki/Fusion_energy_gain_factor#Engineering_breakeven
The fusion reactor must have been built by human civilization or one of its descendants in order to qualify in the context of this question.
Just to give you some ideas about the timescale: ITER, a multinational science fusion reactor is estimated to start 2034-2036. After 3 more years they hope to do some experiments with tritium. It is basically just an experiment to show it is possible to obtain more energy by fusion than you use to start the fusion. The plasma will be only burning by about 1h, not continuous. A commercial prototype will only be achieved by the folow up project DEMO. Don't expect them before 2050.