How much and how often do vampires need to feed? I wish I knew: it would make writing this stuff a lot easier. However enough information has been trickled out so that some informed guesses can be made.
First off, vamps cannot die from starvation, but they do undoubtedly suffer. Spike showed this after he was chipped and could not feed, as did Angel when he was living in the gutters of New York and not feeding properly. It is interesting to note the changes that they experienced. They were both weak: Angel ‘couldn’t go two rounds with a fruit fly’ (Becoming, Part 1), and Spike was definitely staggering around. They also apparently show weight loss. Spike commented that starved vampires become like ‘Living skeletons, mate. Like famine pictures from those dusty countries, only not half as funny.’ (Pangs) And perhaps it provides an explanation for why Angel looked so much thinner in the early seasons (cough, cough) if he was recovering from a century of underfeeding.
With Angel’s trip to the bottom of the ocean we at last get concrete evidence of what happens to a vampire with prolonged and complete starvation. His main symptoms were hallucinations, a weakness approaching coma and marked cracking of his skin, from all of which he recovered remarkably quickly upon being fed. Wesley also stated that ‘A vampire can exist indefinitely without feeding, but the damage to the higher brain functions from prolonged starvation can be catastrophic.’ (Deep Down).
The symptoms that mildly hungry vamps seem to suffer are feeling the cold (of which more elsewhere) and possibly lack of common sense. Although with our boys it can sometimes be hard to tell. Finally, both Spike (Out of My Mind) and Harmony (Disharmony) have complained of hunger pangs in their stomachs.
So they need to eat, but what and how often? Well blood is clearly not just blood to a vampire, or they would all be dining off animal blood and setting up in legitimate business as bouncers and cover girls for anti-wrinkle cream. And even Angel has frequently been seen to have human blood in his fridge, so I would suggest that they have to eat human blood at least occasionally, and certainly prefer to eat nothing else. And vitally when Wesley was trying to revive Angel he decided that pig’s blood was not ‘substantial’ enough and he needed to be given human (Deep Down). There was also the interesting comment that Harmony made that pig’s blood would ‘go straight to her hips’ (Disharmony).
How often, is the hardest question to answer. Angel did once say that if he kept a man alive the fellow would feed him for a month (Dead End). Well, fine. There are about ten pints of blood in a human; a month doesn’t give long to regenerate much so that works out as approximately a third of a pint of blood a day for the vampire. The only problem is that the man would be dead after loosing more than a couple of pints, and two pints for the vampire over the month sounds very little indeed. The only conclusion: Angel was exaggerating (the underhand scoundrel). Back to the drawing board.
The thing we have repeatedly seen is Angel drinking a large cupful of blood; about half a litre by the look of it, which is what the average blood bag contains, so it might be reasonable to guess that this is his daily allowance. After research in my kitchen I can say that half a litre is eighteen fluid ounces, so from our ten pint human that is approximately eleven meals. In other words, to feed the fearsome foursome they would need to make one kill slightly more often than once every three days. Which if you think about the difficulties involved is actually quite a stiff order. Vampires, presumably, must often go hungry.
One last consideration. Blood does not last for very long if not properly refrigerated, even in a cool cellar or cave it would congeal very quickly. Blood is after all designed to clot. So assuming vampires like their blood liquid then, before reliable refrigeration existed, the only way to keep it fresh if you had surplus available would be to keep it alive. And sure enough we have seen both Spike’s gang and Harmony’s with live captives chained up (The Harsh Light Of Day; The Real Me). Modern vamps do of course have access to refrigeration, but they would need a quiet place to drain the blood and bag it so why not have all the fun of keeping a live human? In fact taking one home live for the larder would presumably be a far more normal part of vampire routine than killing on the street. Not a pleasant thought.