How much research is too much research?
'Write what you know,' we are advised. You will interest readers by giving them an insight into your culture. You have a unique understanding of the world you are from. This will delight readers, particularly those who feel guilty about picking up a book for pleasure, and assuage their guilt by reading books that can ‘teach them about something’ – and yes, these people do exist.
Writing what you know is all very well, but what if your culture is inherently not very interesting or indeed too fragmented and undefined to put into a novel? I could write my life story, but it'd be of pretty niche interest as I don't have a home town or a community, I grew up on the dole, and my parents came from disparate social classes and religious backgrounds. You could make one story out of that, and very few people would be able to sympathise with it (although if you do, please comment as I'd love to shoot the breeze).
I suspect most writers share my lack of interesting roots, and so, for almost anyone, research is necessary before embarking on a story.
Writing fantasy is a good way of avoiding research. Just make it all up. Even then, there will be aspects of world-building that require thought - internal research, as it were. You'll need to invent your form of magic. You'll need to be sure it hasn't been accidentally purloined from an episode of Doctor Who you saw when you were eight. And as you're inventing an entire universe, you'll need to make sure the universe is internally consistent, and that you haven’t glossed over something or just borrowed lazily from real life*.
As soon as your story has any grounding in the real world, you cannot hide behind the 'I decided it's like that' mechanism. Sci-fi can involve pseudo-science, but there needs to be some logic to it, and the characters' motivations will be driven by their economic, technological and political reality. Going to the other extreme, could set your story now, in your pocket the world, and it will be dated before it hits the bookshops. You could set it a few years ago, and you'll need to check your facts (no-one is immune to the Mandela Effect. Not even you).
You could travel abroad or into the past, which will involve more web research or even – shock horror – the reading of actual books. (When I wrote a story about a Victorian poltergeist I immersed myself for months in literature written exclusively between 1890 and 1910. That was an experience.)
Research can become an end in itself, as much a thief of time as social media. One tip is to intersperse writing the novel, with researching it. You may find you need to change fundamental plot points because later facts render them impossible, but you avoid the analysis paralysis, the rabbitholitis, of over-researching.
Even if you've spent year on finding out everything about an era, you don't need to use it all. That passage where you go into great detail about muskets? The scene in the dressmaker's shop? A novel isn't an 'A' Level Maths exam - you don't have to show your workings to get extra marks. In fact, research works best when it's so well hidden, no-one even notices it. (My greatest moment of pride was at age twelve when a classmate’s granddad was recorded reading my fictionalisation of the Titanic disaster. My research came second-hand from my mother's recounting of what her neighbour had said about surviving the Titanic. When the recording was played back in Assembly, the whole school thought it was non-fiction.)
Bear in mind that, particularly if you write historical fiction, there will always be readers who are an expert in your era. They will find your flaws. I suggest you make them your BFF and beta reader.
Write what you know, or write what you don't. Once you've done your research, and of course checked it on snopes.com, and checked it again, and checked it from news sources of different political leanings, you’ll probably find the factoid won’t fit into the novel without its being as blindingly obvious as a takeout coffee cup in Westeros.
Sadly, once you've absorbed a bunch of interesting factoids, you are unlikely to ever forget them. I once surprised everyone by knowing the name of the grape-vine disease that decimated France in the 19th century**. Writers are a mine of utterly useless information, and the secret weapon of any pub quiz team.
*Why do aliens so often have two arms, two legs, and a head?