The study I keep going back to for this question is this one:Adjuvant Androgen Deprivation for High-Risk Prostate Cancer After Radical Prostatectomy: SWOG S9921 Study
The study was a comparison of adjuvant ADT2 + Mitoxantrone (chemo) after RP vs adjuvant ADT2 alone. The study was stopped early due to some deaths in the treatment arm (chemo) but the researchers published a preliminary result to describe some unexpected results in the control arm (ADT2 alone).
The ADT2 was 24 months (as opposed to your 18) so you should look at their results for testosterone recovery as a sort of worst case.
In their preliminary results, the study said...
The baseline and demographic characteristics of the 481 eligible men assigned to receive ADT alone are summarized in Table 2. The median PSA of 7.8 ng/mL and highest value of 12.3 ng/mL are relatively low, considering 61% of patients had stage T3 disease or higher, and 16% were node positive. There were 10 major treatment violations in the ADT arm; eight men received no protocol therapy, and two received chemotherapy. The median time to testosterone recovery was 11.7 months after completion of androgen blockade (95% CI, 11.3 to 11.9). The 6- and 18-month recovery rates were 16% and 89%, respectively.