Once, not on a bet or anything, I turned out 300 words on a requested subject in 11 minutes flat. (I had promised no more than 15.)
For someone who writes almost compulsively, the way some people scratch their ass, having to sit down and generate organized words on a specific topic is unbelievably hard for me. Therefore, like any task I find even slightly daunting or off-putting, I am splendid at finding reasons to avoid it.
I think maybe ten of the last fifty Vents were planned more than ten or twenty minutes in advance; a lot of times, I just have to faceplant into the keyboard and hope it makes an impression on me.