I actually stole this topic from a discussion I read…somewhere, a while back. The question was, “when do you stop researching?” I have two answers.
My first answer is never. You never stop researching because everything you read or look at might eventually find its way into your fiction. If you stop researching, I think your stories can grow stagnant.
My second answer is to stop when you have what you need for the story. It’s very tempting to read every book you can find, watch every documentary series in its entirety, read a whole decade’s worth of newspapers on microfiche. And you can do that, if you have infinite time available to you, or are a really, really fast reader. But for most writers’ purposes, all that isn’t necessary; research should be secondary to story, or else no one will want to read your novel. Though they might use it as a research source….
I think there’s a difference between research for its own sake and research for the sake of fiction.
There are two facets to knowing when you’ve done enough. One is that there are things the writer needs to know that the reader doesn’t need to see. I think of a lot of that as preliminary research: reading general books on the time period, making notes of possible items of interest.
After that, specificity is key. (Yeah, I know, I say that a lot.) When I’m writing about World War One, it helps me to know the political background of the countries in which the story is set, but the reader is more concerned with the lives of my original characters, and the details that are related to them. I think of it as a matter of focus.
Research what you’re going to use, as much as you can; I skim through books, marking necessary details with post-it notes, or cut and paste from websites into a single document. I try not to research small items until I know for sure I’m going to include them in the story; instead, I keep a list of details I need to check, so I can search for all of them at once, perhaps on a day when I’m not writing.
I have to admit, I am constantly reining myself in. I buy research books related to a current project that I know for a fact I have no time to read until the novel is finished. I don’t recommend it. Unless your apartment is bigger than mine.