What is the longest interval to proceed with brachytherapy boost for cervical CA after EBRT?

I would proceed with brachytherapy even after a delay of 2-3 months as that is still better than no brachy and if local recurrence occurs, then the patient would need exenteration. Another option to consider, if imaging and scan show great response to EBRT, it is the possibility of a hysterectomy.

In this patient with progression, management changes as now would have to focus on salvaging pa recurrence and not necessarily on incomplete treatment.

I agree with @Dr. First Last. Additional external beam RT to the central disease would have little or no benefit and would increase risks. I would proceed with brachytherapy unless the GTV is very large. If the cervix is <4 cm, you may be able to deliver 50-60 Gy EqD2 to the HRCTV without exceeding normal tissue tolerance limits; this would have a fair chance of controlling her central disease.

I am assuming that the PAN recurrence is outside the RT field. If it is in-field, the chance of cure would be very poor even if SBRT is possible. You may still want to give brachy to prevent a symptomatic recurrence but the emphasis of treatment would focus on systemic approaches.