Let's say you specialize in Ancient Greek history, and live in the US and have zero familarity with, say, Southeast Asia. Supposed for whatever reason you need to get up to speed with the history of, say, Malaysia. Either for the purposes of giving a lecture in class, or for preperation for deeper studies in that area. How would you go about doing so? Ie how would you find and determine suitable books to read, which ones are worth their salt, the general state of Malaysian historiography etc. Also how long would it take to get up to speed etc.
This is how I start researching just about everything academically, history or non. I am a librarian and I only teach information literacy in one-off settings, no history classes. But here's a general life-skill level overview to your question!
Open up Worldcat.
Hunting and pecking, try to find the correct library of congress subject heading for what I want to learn about. Usually not too hard, in this case, it's the wildly creative "Malaysia - History."
Look for a general overview-level academic book published in the last 3-5 years that is widely held in university libraries. (Librarians have already made a general professional call on quality by buying it, if you trust librarians, 1000 copies in Worldcat means 1000 librarians thought the book worth buying for one reason or another.) This means you'll have to pick through quite a lot of results but that's the game!
Work out from here, either working off the bibliography, or going back to the original search and finding slightly older and less general books. I would also start doing basic article searches around this step.
Rinse and repeat for every new intellectual whim! :) It takes me about 3-5 works to feel comfortable in a specific topic, 7-10 + articles to feel good on a broad topic. Time to get up to speed really depends on how much academic writing you're willing to pound through a night.
One way might be to ask a college who specializes in the field. Assuming that is not possible, the easiest way would probably be to search for college or graduate syllabi on the time and era in question, look through the syllabi to see what works are consistently recommended, read those works, other articles by the same author, and articles that the author(s) positively cite. To get more of the historiography, one might try to find review articles in scholarly journals that specialize in the topic.
As to how long it would take to get up to speed, that probably depends on how well researched the topic is (if there are few major works, it might take awhile to find them, if there are many it might take time to properly read and analyze them), whether the research is in a language one can read, and how much you actually want to know. E.g. If you just want a grounding in a particular time and place that is probably easier than getting grounded in the history of a few centuries, especially if there have been major transformations.