I think of it like an accelerator. Even if you're revving pretty high already trying to produce sperm (high FSH) then there's nothing to say that pushing the foot down a bit more might not be enough to produce a few sperm in the ejaculate. If not in the ejaculate then potentially in the testes which can be grabbed by TESE.
My FSH was very high to start with (38 I think) but after arimidex/clomid I still got sperm, so pushing the throttle down even more helped. There's other things at play other than LH/FSH, like inter-testicular testosterone levels, there's a pattern of thought that the drugs increase that level.
To be frank, I don't think the Aussie urologists are all that invested in the drugs approach, it's not profitable. You have to look hard to find someone who doesn't dismiss it, despite the evidence that, in some people, it can help.
Time-wise, it was about 3-4 months, but it can be up to 9 months to truly test it out.