It can definitely take time. My ideas end up coming from times where I randomly dream up a situation which gets me going. Then later on I think to myself that it could actually work in a story. My first story came from thinking up something in my mind to pleasure myself with. My second story was the same, although it was near the end of writing it where I came up with the plans to turn it into a three-parter when I seen an opportunity to make it work. Taken my time on Parts 2 and 3 of my current one to get them exactly how I want them. I also have a couple of ideas for unrelated future stories (again, just by chance when thinking of scenarios which get the blood flowing) but I'm focused on finishing the current storyline first.