Answer

There is no such criterion, and there never was. And certainly nowadays, when there are databases and good indexes, the weight of sheer knowledge keeps diminishing steadily, while the weight of judgment and discernment (which the Rabbinate does not really test) increases. Rabbi Chaim of Volozhin already wrote that in our times there is no obligation regarding forgetting something from one’s learning, and the weight of raw knowledge has declined greatly.
Only once they began institutionalizing rabbinical certification exams (through the Chief Rabbinate) were criteria created, and of course that significantly lowered the level. I do not necessarily oppose this institutionalization; I do oppose treating it as an important and essential criterion, as many do today. A rabbi is a role and a standing recognized by his community / his rabbis / his students. No formal institutionalization is needed, except perhaps for purposes like salary grades and the like. The Rabbinate, of course, uses this to magnify its monopoly, but that is another story.