That reminds me. I had the craziest dream last night. It would have been a great source of plot ideas. When I woke up, I promised myself I would write it down before I forgot. But there was something I had to do, and I guess I just forgot to write it down. Now I have no idea what the dream was about. Fuck me.
It depends on what you mean by 'plot.' A lot of people say 'plot' when they mean 'idea for a story.' Developing a plot takes me anywhere from three days to three weeks using tools from Swain, Blake and Dramatica. Ideas for stories, on the other hand, just pop into my head and I might be anywhere doing anything. If they're still there the next day, I write them down (King).