Hi Andrew,
I have been asking myself the same question.
This is just one bit of anecdotal data, but for what it's worth, I started like you with a month on Bicalutamide, then 3 monthly Prostap. That started in June 2020. Starting PSA was 17.9. To my great ( subsequent) annoyance I didn't get a starting Testosterone number.
At the 3 month point, PSA was down to 4.9, and looking good, so I had the next 3 month injection.
By December, the trend had done into reverse: PSA was up to 10 at my 6-month review, when I should have been planning for my RT. Testosterone was checked at that point and came in at 7.9- so a long way off castrate level. It was concluded that the Prostap was not working effectively, so the decision was taken to switch to Zoladex. Unfortunately - second source of regret - I had already had my second Prostap jab, so had to wait for that to wash through. ( As an aside, Onco guy was helpful in arranging an MRI scan at this point to set our minds at rest that there had been no physiological change in the tumour).
Zoladex started in February 21. PSA was up to 13.9 but Testosterone down to 5.9 ( doh!).
I have Just had my first 3 month review since starting Zoladex: Testosterone is right down to 0.4 ( job done) and PSA down to 6.1( better, but no coconut). Onco guy is expecting it to move ( down) relatively quickly now - so my next review is in 6 rather than 12 weeks.
Lessons learned: get PSA AND testosterone tested at the same time; and get blood tests done well before the next jab date in case a change of plan is required....
All the best,
Ron