Prostap failed for me after a few months ( PSA doubled in just a few weeks) so I was switched to Zoladex, which did the job. Even so, it took a second blast of Bicalutamide to get my PSA down to a decent starting point for RT.
As Lyn says, definitely keep an eye on testosterone levels - I think the absence of checks in my early days allowed the Prostap failure to go unnoticed for a while. After that I always had the testosterone added to the blood test