I've read somewhere a piece of advice or a guideline regarding non-fiction, which suggests that when writing, you should convey 10% of your knowledge on the subject, and for every final page of text, you should have about 10 pages of notes. I believe this also applies to fiction. Recently, I started writing a cyber-fantasy novella set in Hamburg (I live nearby, and it's a bit of a tribute to this extraordinary city). To write a single page, I had to spend hours with a map and visit the locations where the action takes place.