When her actions are consistent over a long period of time.
We get this question a lot. The best answer is that when she is really ready to commit back to the MR you will know. One of our regulars shared the quote: "When they want to reconcile, you will know. When they don't, you will be confused."
But when you know you are no longer Plan B is when she is consistent in you being her Plan A over a long period of time.
I know in my sitch, I wondered the same thing. The truth is it won't be a moment where you go: "she seems like I am Plan A starting now!" What happens as her behavior shifts to support that, you will at some point look back and realize that she has been consistent in her actions that you are now Plan A for a period of time. For me it was like 3 months into Ring that I realized she had been consistent in her actions.
Anyway, I wouldn't worry about that. Her actions are out of your control. Keep the focus on yourself and you actions and you will be fine no matter what she decides.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018