Any genre, time period, area, or culture that you are unfamiliar with will require research to write in it believably. Even areas you already know may need additional details that you have to look up. Overall, how much research you need to do depends on what you’re writing and your target audience.
If you’re writing for an extremely educated audience or more lofty circles, you have very little room for error (especially if it’s nonfiction). Your research had better be very in-depth since glaring errors will outrage your readers and lose your audience. They may allow a couple errors of semantics (technically, the granary would have been on the opposite side), but that’s about it.
If you’re writing for the general public with the main goal of entertaining, you have more leeway. Most people won’t recognize little mistakes, and they’re only interested in the information as it relates to telling the story. If someone has specialized knowledge (like they live in the town you only know through research), they may smirk and laugh at your errors. At the same time, they’ll be intrigued and excited about everything you got right. Unless most of what you wrote about the town is glaringly wrong, they probably won’t stop reading because of the research.
So how do you know how much is enough? Well, most research is really a kind of worldbuilding. Like worldbuilding, you need enough details to be interesting and to make the story believable. You also need whatever details of setting, character behavior, and technology are required as part of the plot. These details need to be accurate, and if they vary for some reason (for example, a character does not follow society’s norms), then you have to show that you know the character is not behaving normally.
As a general rule, make sure you get the most famous or most well-known details correct. If your story is set in the U.S.A. in the fall of 1942 (during World War II), and you have people baking sweets and using sugar in their tea constantly, you’re going to lose any readers who know about the war rationing. If you have a woman becoming a Navy SEAL in that same story, you’ll lose even more readers. If you mention a song that actually came out in 1943, most people aren’t going to know – unless it was a major event.
As hard as you try, you will probably make mistakes. The main point is that many readers will be forgiving if they can see that you tried. Get the big details right, and they’ll let the little less-known facts slide. Yes, you still have to research and put in some elbow grease, but for most authors, there’s no need to obsess about every tiny detail. Save that energy for the story.