I'm kidding. You can never have too much research.
Yes, fine, eventually you need to stop researching and start writing. But most writers stop researching far too early. There are three good reasons to take the time to do your research.
(1) You should never be wrong by mistake.
It's not just that if you are writing a historical novel you have an obligation to get the history right, and if you are writing science fiction to get the science right. Actually, you don't necessarily have to have accuracy in these things. (I know, I'm shocked I said that too.) The thing is, the story comes first, so yes, if you need to pretend that your imaginary early modern British coal mine already had a Thomas Savery steam engine in 1675 because you want it to coincide with the Great Fire of Northampton, go ahead, just so long as you know it wasn't patented until 1679. Don't get the details wrong out of ignorance.
In The Unfinished Song, I mix the flora and fauna of several continents. It was a deliberate move on my part, because I didn't want my story to be considered merely "Celtic" or "Viking" or "Native American" or "African." It has elements from all of those cultures, so I included elements from all of those places. Maybe if I were to do it all over again, I'd do it differently, but it was a conscious decision.
(2) A well-researched novel is less derivative and trite.
Have you ever read a book or watched a tv show and felt like the setting was "thin," or you'd seen these cardboard characters a million times before, or guessed the "surprise" ending from the start? There are a lot of reasons for thin setting, shallow characters and poor plotting, but one reason is that when we are visualizing a scene in our mind, we are limited to what we know. Orson Scott Card warned about the danger of a writer using the first idea to pop into your head. Chances are, this "inspiration" comes to you so easily because it is trite...it's something you've seen a hundred times before in films and books. That's why it falls flat when it goes into your book.
When you do research, ideas also pop into your head, and they might also follow closely on what you've read, but, if what you are "copying" is an original source, your fiction is ironically more likely to feel fresh. Let's say you're writing a regency romance. If you read only other regency romances, your book will read like a copy of a copy. But if you read a book of real letters from a regency lady, and let that inspire you, you are more likely to capture a unique and authentic voice.
(3) Research primes the pump.
The main reason to do research is that it is a wonderful source of inspiration. Whenever I feel writer's block...now, for instance...I do one of two things: (1) read other people's novels, (2) research. In fact, the number one reason I suffer writer's block is because I don't know my world and my characters well enough. Research isn't just about dry facts. It's about spending time with your characters, moving around in their space. But dry facts, believe it or not, can help you do that. Consider something as simple as handwriting.
I used to work for a national magazine and write an advice column. I received hundreds of letters and I could always tell how old the person was by their penmanship. If they were over 75 they wrote in the Spencerian method. It is a cross between what we would call Cursive writing and calligraphy. If they were over 85 they would write in the Spencerian style with a little bit of a quiver in the letters. ( see picture above)How would knowing this help you? It might be of major import. Suppose you are writing a mystery, and the kind of handwriting in a note left in a coat pocket is a crucial clue. But this knowledge could also just help you know more about your characters, to describe the elderly aunt's slanted Palmer letters and her young nice's blockish, txt-like handwriting.
If the writer was between 65 and 45 they wrote in cursive writing of the Palmer school which is a slanted writing with specific style to each letter.
If they are under 40 they wrote with a smidgen of cursive style and a smattering of printing.
If they are under 30 they often wrote in cursive and when they became anxious or tired by the end of their four line letter they often fell into block letters.
If they are under 20 then the writing is usually block lettering and then becomes larger and more scrambled when they are expressing an emotional thought. It often looks like the writing of "Son of Sam",the paranoid schizophrenic who wrote to the newspapers in 1976.