Already posted this on another thread.
Basicly - Any endogenous test = sperm, A little exogenous test = no sperm, a lot of exogenous test = sperm
Testosterone stimulates the Sertoli cells in the testicles to produce sperm: It is the local production and presense of testosterone that produces sperm. When one injects a small amount of testosterone, enough to signal the leydig cells to stop producing testosterone, this local presense ceases because by the time the exogenous testosterone reaches the testicles it is in such a small amount compared to the locally produced test. Now when one injects a high enough amount, like 750mg per week, there is enough test reaching the testicles to stimulate the Sertoli cells; even though your leydig cells aren't doing anything to produce local production, the sertoli cells are still being bathed with testosterone. This is why 100-200mg is the dose used for birthcontrol.
It takes about two weeks for the hypothalamus to stop producing GnRh, it takes at least longer and up to 8 weeks for the leydigs to shut down completely, so one would be looking at anywhere from 2-8 weeks for it to work. There are two seperate levels of surpression.