One of the questions those contemplating writing historical novels usually ask is, “How much time do you spend on research and how much should you include in a work of fiction?” There is no typical answer. The audience, type of novel and how much a writer already knows about a given subject will determine research hours.
Elizabeth Chadwick, a writer of medieval English novels, clearly spent quite a bit of time researching the food, clothing, political climate, customs, expressions and language of the period for her first novel. Presumably, she spent less time researching her second, third and subsequent novels all set around the same time, hence, the benefits of sticking to what you know.
Then there’s the strategy of going so far back into prehistory where so little is known that you can write just about anything you want, within the bounds of credibility, of course, hence, Jean Auel and the CLAN OF THE CAVE BEAR series.
If the characters are actual people from history, more care must be taken to get them right which is why many historical romance authors create fictional protagonists with real people having small, walk-in parts. That isn’t to say that license can’t be used with actual historical figures as long as it’s indicated in some way, usually in the forward. This prevents history buffs, who know more about certain subjects than the author, from writing scathing reviews because Margaret Fitzgerald’s hair was brown, not red, or Mary Queen of Scots never said anything of the sort because she could only speak French and was allergic to lavender. Joan Wolf does a wonderful job in her King Arthur book when she throws legend to the winds and creates a young, compassionate Morgan who falls in love with the sexiest, equally young, King Arthur ever created, and he with her.
The best piece of advice I've ever heard when it comes to research is: note everything you could use but include only ten percent of it.