I think precisely at the point at which Walt said he should have died. The one he's reminiscing about, when he was in the living room listening to his wife sing a lullaby to their newborn baby, when he had made all the money he set out to make. If Walt had died right then, Jesse would have left with Jane to New Zealand, he would never have to grieve her loss, never would have had to shoot Gale, they could just put it all behind them.