How much is enough? Is there ever enough?
A thirst for blood is arguably THE defining trait of a vampire. They drink it to exist and helpful humans are readily available; what varies from story to story is the actual need.
The first consideration is what the blood is for. In a modern twist, vampires may be portrayed as biological, needing blood due to an inability to manufacturer their own or requiring some essential element that only living blood contains. In such cases, the vampire may be susceptible to blood diseases or the effects of substances such as drugs or alcohol. For the more traditional “mystical” type, “the blood is the life,” allowing the vampire to literally take the life force of the living into themselves to empower an animated corpse.
In either case, how much is enough? How long does it last? How often must the vampire feed?