All fiction, regardless of genre, requires some research. This may be choosing a spot in the middle of the Pacific to create your own island nation, discovering if it’s physically possible for a plane to fly five times the speed of sound, or finding a common Scottish name for your hero. There might be small searches, or you might need to learn the entire history of a particular group of people.
If like me, you didn’t pay much attention in history class at school, there is even more of a need to research as you have less information to use as a starting off point.
But when to know when you have researched enough?
The key to answering this is not in the timeframe but in the research itself. It is impossible to know everything about every subject related to your fiction manuscript. Instead, when researching, you need to read through the lens of your manuscript. For example, what were women’s lives like in small villages two thousand years ago? Or, how would a strong yet stubborn hero have participated in the French Revolution?
By using this technique, you will read only what is relevant to your current manuscript. And from here, it is simple to know when to stop researching; you stop when you have just enough background understanding to start writing. You do not need to be an expert, you only need to know enough to be credible. To research any more than this is procrastination.
Sometimes, you will need to research for weeks or months, but it should all be in the context of your current manuscript and not a page more.
Remember, the research is there only to give you ideas for setting, background and possible plots lines (or holes which need fixing). Your story will not be found in that pages of a non-fiction book, so use then only to answer the bare essential questions.