My experience (over 25 years) makes me think that:
1 If you get very little change on HRT (boobs etc), from perhaps a low dose, then you will get only a slow loss of function. Difficult to quantify but I would say about 50% per five years.
2 For rapid change, the function loss can be very quick (in 2 months on E alone).
3 Even when the loss has been complete, Ciallis still works but it is an effort. Viagra not so good.
4 If you start off with a low sex drive, faltering erections, then the change will be quicker
5 If you stop HRT then function does return and takes about the same time as the time you were on HRT
6 Sex with HRT+ ciallis is completely different (better for both of you)
7 As much as I enjoyed sex, which was a lot, I love life without T.