From the blinking experiment after someone’s head was lopped off, probably at least half a minute?
On an unrelated note: as a sophomore, I was asked by my history professor to draw a cartoon for the front page of the Western Civ final history exam of the semester, related to guillotines. Which I did. And all students started giving me the side eye as the proctor handed out the exams….
We’d expect that the heart rate of someone being led to their guillotining would be at a near-tachycardia level (unlike the auto accident decapitation victim described in Cecil’s linked article - knowing what’s coming makes all the difference, and supplies all the societal revenge no matter how painless the method), pressurized sufficient to flush the oxygenated blood from their brain. Also the WHOMP of the blade would be similar to though immensely more than the sensation of hitting your thumb with a hammer: the pain doesn’t register for moment or so. But in the case of the guillotine the victim is in the blackness before the moment passes. Dr. Guillotine may have been correct in describing it as only “a slight chill on the neck.”
The central nervous system is, well… decapitated. The “mini-brain” that regulates the heart will go on firing for a bit, but it’s not a thinking brain. The peripheral nervous system: sensory and motor neurons, may well go on twitching and feeling the splinters in the plank and the snugness of the straps, but no consciousness receives them.
One of the more interesting scenes in the 1981 movie Wolfen (based on the novel by Whitley Streiber, before he went Cuckoo for UFOs) comes after they have a discussion of just this issue. One of the people in the discussion gets decapitated, and you see his eyes blink and mouth move, indicating that he lived and was conscious for a few seconds afterwards.
This doesn’t say anything about what happens in the Real World, but it was a cute touch in a horror movie.