I loved this post by Robin Hobb on her Reddit AMA:
Daydreaming? That’s almost my full time occupation! But from the time I get an idea to the time when there is enough of it to say, “This will be a book and it’s time to start writing” is usually at least a year, and often much longer. Many times I’ll get a tiny gem of an idea, and I love it, but it’s not really enough to be a whole short story, let alone a book. So I set it aside and wait. And other bits of it come to me, dialogue or names or settings, and I add that to it. And Wait. And what usually happens is that I’ll be perusing the story seeds, and I’ll suddenly see that two or even three or four are all parts of the same story. When you put them together, they start striking sparks off each other, and growing. It’s wonderful. Then it’s time to write the book.
Everyone has a different process, but this really struck a chord with me. I’ve found that there is a point when an idea is ‘ready’. I sit on it, mulling it, rolling it in my head on car drives, walks, and really boring meetings (if I’m in a long teleconference for you, it’s probably a space opera playing in my head. Sorry.) And you just know when something is missing. But every once in a while, another idea buzzes in, clicks, and a golden link is formed.
Of course, I’m much lazier so it just gets filed away in my notes for later. I have a lot of notes.
Incidentally, Robin Hobb is an amazing author, and you should check her stuff out if you haven’t already.