I have been training and practising retention both with mtb and with sex for many years, since the year 2000.
Now, I can have sex for 1 hour no problem, without ejaculation, and if i have the energy I can go longer. Also, if my partner does not complain that "this is too long", "i am tired", "OMG, you are having sex too long time" etc, maybe I';d have a field day.
I have had girlfriends, and most of these girls are like tired after 15 minutes of active sex. Ok, with repeated training with me, they can slowly go longer time.
I have tried doing it with escorts. Once, I went for no ejaculation for 1 month. Every 2 - 4 nights I would have sex with a young girl. At the end of the one month, I found that with each retention, I could do longer than the last session. At the end I felt like it became a chore or task...
I have only met 1 or 2 women that can still enjoy sex after 30 minutes of sex.