Not to be trite or over-spiritual, but I remember hearing an account of a son hearing his father preach, and afterwards asking, "How long did it take you to prepare that sermon?" His father's response was something to the effect of: "My son, it took my whole life to compose that sermon." I do find there is truth in that. I think many of you will know what I mean.
I also think that since we're all different there's not necessarily any true rule of thumb here. Spurgeon both chose his texts and put his sermons together, I believe, Saturday night. Not all of us are Spurgeons. For some it will take longer. For others that are really gifted with preaching, it may come faster--but if it does maybe that means they need to be spending the extra time soaking it in prayer.