The particular issue of quantity consumed is never addressed, in-universe or out. The twin does literally devour the flesh (and, consequently, much of the blood) of the "victim". However, since this happens during the pregnancy, the fetus is likely autonomous (for the most part), and thus would consume it in its entirety.
This phenomenon is known in real life as "vanishing twin syndrome" (Wikipedia - warning: graphic medical images). There are cases of both partial and complete consumptions.
Consequently, because of the real-life nature of this issue, it has been seen throughout history. Most primitive cultures believed that twins meant something spiritually, even in terms of magic and righteousness.[ref 1] Some thought fraternal twins were the sign of adultery, and identical twins were the sign of divine paternity.[ref 2, pg.1, paragraph 3] Often, in older societies with high infant mortality, only one twin would even survive the birth. To explain this to the surviving twin, explanations of magic and myth were often used.[ref 2, pg. 2, paragraph 2]
Why this is important
I know what you're thinking: "Does this relate to the question?" It does, because it somewhat shows the roots of this concept in Vampire Knight. Twins were often lost before they were born, and so legends came to be about why and how this happened. One such legend is that two identical twins are, in fact, one being which has separated. In order to regain their full strength, they must be reunited—one must be absorbed.
So, how much does the strong twin have to eat to become whole? According to the legends, it only has to be enough to kill the weaker twin, so that the life force can be absorbed. Let's say it took a small bite from the arm of its victim—that would likely not kill it, and would do nothing overall. But eating, say, the head or heart (or the entire body), would be sufficient for the victim to die. Assuming these legends are the roots of the concept in Vampire Knight, the same logic would apply.