We all know that food spoils if left untreated by some form of preservation. Different foods spoil at different rates, of course.
Let's say Dracula stumbles across a fresh corpse. (For the sake of argument, assume the person died of natural causes.) How long will it be safe and effective for a vampire like him to drink the blood of that body?
(See this question for a similar question about the nutrition of preserved blood. I'm instead asking about unpreserved blood.)
For a starting point, consider the meat aisle at the grocery store. In it you'll find roasts, steaks, and other chunks of red meat. This government food-safety site says that such meat can be kept in the refrigerator (40 deg F) for 3-5 days. That's for meat that's already been cut up, drained of most of its blood,1 and packaged for individual sale. Before that, it could have been kept for a couple weeks or more. That's just some context; stuff that came from dead livestock, in general, can be safe for rather a while.
That's for meat that people are trying to keep safe. But that's not Dracula's case; he's not raising livestock. So while bodies kept at the right temperature could supply some blood for a couple weeks, it's bounded by how long a body not kept at the right temperature can sit around before the blood is dangerous to consume. We also have to consider coagulation -- how long until the blood isn't sippable any more because it's clotted?
Spoilage due to bacterial infection in the blood can begin within hours, but appears to come from two main sources: bacteria in the intestine at the time of slaughter, and infection caused by the slaughtering process. Indeed, one government agency has special guidelines for collecting blood for human consumption (17.6.3.1.1 ). Dracula should select for bodies that died intact, not roadkill or ones that died from wounds, to minimize risk of infection.
As for coagulation, if there has been an opening (like an incision), blood coagulates quickly after death (within minutes). If there is no wound, however, blood remains liquid for several days after death. (See also the Wikipedia article on lividity.) That's true even at room temperature. (Freezing does bad things in the coagulation department, but that isn't Dracula's situation.)
Putting all this together, an intact body with no special precautions will contain liquid blood for several days. If it is kept cool during that time it should be safe on the infection front. (Plus, maybe vampires don't worry about infections; consult the physiology specifications for your particular model of vampire.) If he cares about blood-borne infection, Dracula will do best in cooler climates, whether natural or man-made (morgues).
1 I'm going to draw on an unusual source for this: Jewish dietary law. There is a religious prohibition against consuming blood. When an animal is slaughtered its blood is drained in the usual way (just as it is for other slaughter), but Jewish law requires an additional process of salting the meat to draw out more of the blood. It's of course possible that this is completely redundant and what the rabbis who instituted this were seeing wasn't blood but water with myoglobin (thank you Selenog for teaching me about myoglobin in meat). But I've heard a lot of arguments over the years about how such-and-such practice shouldn't be required any more because science, and I have never heard an argument against this one.
It is Not (for some mythologies)
It should be noted that the blood of the dead is poison to vampires in a number of mythologies and stories.
For example in interview with a vampire.
! Claudia poisons Lestat by tricking him into drinking blood of the dead. "She tricks him into drinking the blood of twin boys she killed by overdosing them with laudanum, knowing that blood from a corpse is fatal to vampires. This weakens him, and she slits his throat. Claudia and Louis dump Lestat's body in a swamp and the two plan a voyage to Europe." --https://en.wikipedia.org/wiki/Interview_with_the_Vampire_(film)
And in Supernatural the Winchester Brothers use blood of the dead against vampires.
Given you tagged this as a 'vampire' question, I'll assume you're a friend of mine and we're chatting over tea ^_^ (actual tea, not blood in a teacup)
You have to consider what the vampire is drinking the blood for. Yes, TrEs-2b is right on the medical facts (I didn't fact check, but it sounds about right). However, this isn't just about the medical, this is about the mythos of the vampires you're talking about.
Now, seeing as our vampires are different I will assume you are going to change certain things to make them make sense for what you want them to represent. If it's a magical thing, and they are 'feeding' off people for (insert reason here), then you'll want them to be alive as they feed. If you want them to show you the 'lawyer stereotype', where "they'll still screw you after you're dead", then go with the medical maximum (TrEs suggests about half an hour, but keep it shorter because you don't want 'oops moments'). Or if they are meant to represent an undying link to the past, then it really doesn't matter -- you can have them drink week old, dried blood, just give them some kind of handwavium to make the blood 'fresh from the vein'. Because it's about their preservation, not about the blood itself.
The point is, you need to focus on what your vampires represent, not what their limitations are. Yes, you need to know the medical/scientific limitations, but limitations can be worked around. Not only because this is the evolutionary concept, but because this is your creation.
Dracula and his ilk are supernatural monsters. Sorry, if that's offensive, but how else can you describe someone who is an undead bloodsucker. So it is doubtful if health concerns such as infection will harm vampires. Therefore, the most likely limit Dracula faces is how long blood remains fluid, so he can siphon blood from a cadaver.
Research into the topic of blood coagulation proved harder than expected. Certainly a few hours after death is likely, but longer times may be possible. Monica Cellio has suggested longer times, but I am not confident about whether this is either right or wrong. The information isn't readily found or what can be found is difficult to decode into something useful for this question.
Considering that vampires have a tendency to prey on living victims this does suggest that they may be the kind of predator that preferentially attacks live prey. In which case, the time would be zero. Once dead, someone is no longer vampire prey.
The other alternative is rather yucky. Perhaps because vampires are supernatural monsters, their supernatural powers extend to draining blood from even the most dead of the deceased. This means the long dead, the sort who get put into the ground. Basically the ones who haven't started to decompose yet. Considering vampires rest in graveyards that receive regular supplies of the newly dead, so they might preferentially feed on these new corpses. It would save going into town and molesting live prey. There will always bring the blowback of people wanting to stick stakes in the hearts of the vampires.
Well, basically you've got two forms of Vampires when it comes to blood: Ones who derive sustenance only from blood, and those who only drink blood because it contains the life force of the victim.
The actual blood would be divided into Plasma and Blood Cells. Plasma would last until it dries out, because it is basically a bunch of dissolved nutrients. Blood cells live 60-120 days in the body, but the actual cells would live until they exhausted all of the glucose in the blood stream (don't need oxygen). If other organs are to be judged by, anywhere from 4-48 hours.
If you're talking life force, that would be anywhere from the time of death to 3 days after (based on Hebrew resurrection lore). Depends on the system.
Depends probably on vampire taste and physiology, since many novels assume vampires have great smell, than probably they only like blood from alive meals because of the smell of adrenaline/fear.
The main point is that animals' blood is good for human usage after several hours (depends on temperature, in a hot summer I would assume half-hour, while in winter is probably fine up to 8 hours). But that is based on human physiology. When you look deeper you will find that blood is made of several different compounds, each one taking a different time to decay according to the temperature/light/pressure conditions. Wich compounds do vampire need to consume?
Maybe blood temperature plays a role, since body temperature decrease by 1.5 degrees each our, then maybe a vampire cannot consume blood cooler than 33 degrees, so it has to drink it after 2 hours from the death, and probably cannot drink blood from alive targets in hypotermy
I imagine as long as it would take for blood to dry, half an hour-hour. A study published in the International Journal of Legal Medicine reported that a blood drop on a hard surface in a typical indoor setting at 20 degrees Celsius is completely dry in 60 minutes. Increasing the temperature to 24 degrees Celsius reduces the drying time to only 30 minutes.
For Dummies says that deoxygenated blood enters the right atrium of the heart during circulation, at which point it is forced through the pulmonary valve and into the pulmonary artery. Here, the deoxygenated blood travels through the pulmonary artery to the lungs where it receives oxygen and becomes oxygenated blood again. From here, the process starts all over again, and the oxygenated blood leaves the pulmonary artery to bring oxygen to the cell membranes once more.
I am assuming that vampires can consume blood off a dead person (which is not always the case as mentioned in user3559247's answer). It seems likely (to me at least) that vampires do not actually suck the blood out entirely by themselves; instead they would partly rely on the heart to pump the blood out for them. A human heart can keep pumping blood for a maximum of a few minutes after death, for example until it fails due to lack of oxygen.
Of course the whole things depends on how you define death. With the heart still beating, some might say that a person is not dead yet. But it seems reasonable to say that a person whose brain is smashed beyond any repair is dead, even if the heart would still keep beating. From such a corpse a vampire could quite easily suck blood until the heart stops beating.
There is another thing to consider also, and that is blood clotting. It is probably safe to assume that a vampire cannot consume coagulated blood. I couldn't find any sources on how long it takes for blood to clot after death; but it does depend on the type of damage on the corpse. If there is plenty of damage to the blood vessels, the blood clots quite rapidly (in a matter of minutes!) at least around the damaged portions. Without damage, maybe a few days until the blood stops being a liquid?
As a final note, in the time frame between the stopping of the heart and blood clotting, the vampire could use gravity to get the blood out of the corpse, as a butcher does with an animal carcass. But note that butchers usually drain the blood immediately after death or stunning of the animal, as the blood comes out much easier with the heart still pumping.
You could use forensic science data on post-mortem lividity (pooling of blood in the lower portions of the body, and its loss of oxygen) to set a time scale. This website say that the process is well developed in 4 hours after death and a its maximum at 6 to 12 hours.
Or this website says that after a few hours the blood becomes fixed in place. So Dracula would have to chomp and chew rather than sip and suck to get a meal!
A talented, experienced person (like a butcher or Dexter) could do quite well at draining a fresh body.
Let's say your Dracula is one of those and he stumbles across a freshly killed mortal within minutes (less than 30 minutes) of the unfortunate's demise. Dracula may, depending on time and equipment, be able to drain as many as 10 pints (on average) of blood from the deceased. That blood, if kept the way the American Red Cross keeps it (refrigerated and in an airtight seal), can last for as long as forty-two days.
