The course that taught me the basic outlines of the historiography of the medieval west involved reading approximately 60 books and 30 journal articles in three languages, and took place over 4 months. This course was considered preparation for one doctoral general exam, of which three or four are required to be allowed to continue on to working on a dissertation.

I've read about 25 - 30 books, plus other sources, to become an "expert" in one single aspect of a 20-year period of Australian history that happened only 110 years ago. This includes primary sources like books written by people who were in the thick of it, and legislation, and government reports, and so on.

If one wanted to become an expert in something broader like... for example... jobs in ancient Rome... one would have to have read a lot of books and articles and primary sources. Also, employment in Ancient Rome is a much broader area of expertise than the federation of the Australian colonies, and the temporal distance is orders of magnitude greater, making it that much harder to learn about. There's also the fact that ancient Rome existed for over 1,000 years, from small city-kingdom to republic to empire. One would have to study for years and years to even start to cover the whole of that period, or one would need to choose one very specific limited time period to focus on in-depth.