Few minutes (4 to 5) of anoxia (lack of oxygen supply) are sufficient to cause permanent damage to neurons, compromising a brain's functionality.
On average, the brain can survive a mere four minutes without oxygen, so if you suspect a stroke or other injury that is cutting off oxygen to the brain, you need to act immediately.
As easily as 30 seconds in, brain damage can begin, and at two minutes brain damage becomes almost inevitable.
With the neuron gone, gone is also the capability of releasing neurotransmitters and electric signals, which is how the collective behaviour of the brain neurons emerges.
Since with death also the respiratory function ceases together with the blood stream and neurons are the first to be damaged, it's likely that your technique will read increasingly noisy signals the more minutes are passed from the moment of death, until the read out just becomes a white noise.