Good Question
Very good question,
The answer is "it depends".

Testicular atrophy will occur anytime the HPTA believes the body is getting higher than normal levels of testosterone. The HPTA signals for a reduction in LH and FSH, which causes the lydig cells to slow production. Anytime the testes start to produce less, they start shrinking.
However, this will depend on dose. When you start TRT, you will get a blood test to determine your T levels. If you are in the normal band, you will most likely be placed on 200mg/week of testosterone cypionate. This level is only likely to cause a small amount of atrophy over a long period of time.
If you are in the lower range, you will likely receive 400mg/week. Again, since the body is already at a lower level, it is doubtful you will get any significant atrophy.
Once you go past what your body thinks is normal, you will get the atrophy....however that is going to vary from one person to the other, vary based on dose, and vary based on length of cycle.
If you do notice atrophy, it is not Nolvadex that will help you. Nolvadex is an anti estrogen that competes with E for the receptors in the body. This prevents gyno, weight gain, moodiness, etc. What you need for atrophy is HCG, or human chorionic gonadatropin. This mimics the bodies own production of LH, which causes the testes to fire back up again.....allowing them to come back to regular size. However, HCG does not to normalize the HPTS, so you will need Clomid therapy once you stop the T and HCG.
It really isn't as complicated as this last paragraph sounded. If you need some more exact dose, etc....let me know.
Hope this helps.....
Kemo