It’s useful to know how long it takes to make sperm in the testis. Based on the brilliant work of Charles Leblond and Yves Clermont in the 1950s and 1960s studying how testes looked under the microscope, scientists guessed that it took about two and a half months to make a human sperm. But Leblond and Clermont were looking at specimens that weren’t actively making sperm.
In 2006, Paul Turek did a clever experiment. He gave men radioactive water (not enough to be dangerous, of course,) and waited until their sperm became radioactive. In most men, it took about 60 days. From the moment a sperm sets out on the testis assembly line, it looks like it takes about two months before it reaches the end.
So if you’re taking a medication like clomiphene that works on the sperm factory, expect it to take about two months to begin to work.