I'm absolutely certain it depends on the person, because all hormonal stuff varies by individual.
For me, I'd been on HRT just about 2.5 years when I had GRS, and as part of the surgical procedure the doctor sent my testicles to pathology. The report (which I got my hands on months later) stated that they were "normal sized, but nonfunctional." So I, at least, was permanently sterilized and castrated by even low-dose estrogen-only HRT. I wasn't even taking an anti-androgen; I never have!
(I also had to go off of HRT for three weeks before surgery, and had no real symptoms, aside from a couple of minor hot flashes.)