For Hillary, it is whenever she wants to jump in. Up to (and maybe even after) the Democrat convention.
She will come in at the last minute to "save" the party. That way, she won't have to go through the grueling primary process.
Hillary never plays by the rules, and they let her.
Once again;
She spent 1.4 billion dollars and still lost a rigged election to a political nobody.
She will never be a candidate for anything ever again.