That's only possible in cases of very limited braindead. "Mostly" a braindead person will have a slowly lowering heartbeat, and once it hits 20 or so, they die.

That's only possible in cases of very limited braindead. "Mostly" a braindead person will have a slowly lowering heartbeat, and once it hits 20 or so, they die. This usually takes hours, not days. Now, there's a gazillion different ways to be braindead. It can be that the spine is disconnected somewhere (in fact this is the most common reason, thank you traffic), and on the other end there is no sharp cutoff between braindead and in coma, where we see no damage whatsoever and can even be brought on by a normally unrelated disease (but often some form of poisoning is suspected). Therefore there can be very weird phenomena in a significant number of cases. Also the symptoms depend on the exact point the disconnection occurs. If it's deep in your back (10th vertebra or lower), you're dead in minutes, unless it's very, very deep (18+). But mostly, you don't survive braindead for very long. Even an artifical "braindead" state (which is what barbiturate overdose patients die from) normally doesn't last more than 3-4 hours before death occurs, including the period where activity slows (so starting from the point the overdose was administered).