AC is right, the response can be so different from one person to another.
For what its worth, I can tell you my experience with Decapeptyl has been disappointing. But that may not be the case for your dad. I had been on Degarelix since my diagnosis in 2015, but I did suffer bad reactions to it. It worked for about 15 months, but then it had to be supplemented with Abiraterone. The combination worked for about 10 months and then my PSA started rising again. I started Docetaxel chemotherapy in October 17, but the combination between the Docetaxel and the Degarelix was just too much and so the onco changed the Degarelix to Decapeptyl in December. But my PSA continues to rise, doubling between Feb and May.
I am sure your dad's onco knows what he/she is doing and will have made an informed judgement in setting the frequency of his appointments. But I hope everything works out ok for you.
Best wishes
Peter