If you had your head chopped off how quickly would you actually die?

Your brain would presumably have some blood already in it, so wouldn't you conscoius for a few seconds? Would you be aware that your head had been separated from your neck? I think about the French Rev sometimes and they've gotem lined up to go "chop-chop". How long it must seem for that blade to come down. Then, wouldnt there be just a few seconds of being aware that your head at just come off and is falling into a bucket? A question for the Sweeny Todd fans among us... :-)