How long does a beheaded human head live?
Taken mostly from
New Scientist, The Last Word, 16 December 2000
An account from a Dr Beaurieux, of an execution by guillotine states that 5-6 seconds after the event the lips and eyelids worked with "irregular rhythmic contractions". After this, the eyes opened and rolled back. Calling the victim's name caused the eyes to move back into position and focus on the doctor and then go back to how it was. Calling a second time had a similar effect, a third call had no effect. The whole account describes 25-30 seconds of events.
There are accounts of victims of the French Revolution being asked to blink their eyes when they were beheaded. They are supposed to have done this for up to 30 seconds after but it is not sure whether this is mortal spasms or not.
I am not sure how much of this is possible. If you think about it, how long can you survive if your head blood pressure suddenly becomes zero?