Blood Rage (3.5e Flaw)
From D&D Wiki
Blood rage can only be satisfied through drinking blood of the living or recently dead(no more than 8 hours). The blood needed to sate the rage is equal to that found in a creature one size category smaller than the you. It takes two creatures of a smaller size to equal the next size larger. For every hour of blood rage, the character must consume that much again to satisfy the lust. So a human who has raged for 3 hours would have to drink the blood of four halflings or two humans. One small creature as the base, per day requirement, plus one per hour.
The amount of blood left in a creature after death is variable base on whatever your DM decides but here are some general outlines. A creature who died mostly (let's say 75%) from bludgeoning and energy damage still has most of its blood and is valid as its regular size, however if the creature died from piercing and slashing, treat it as one size smaller. The special exception to this is if from a bite attack by the blood raging individual.Consider the damage done from bite(may be an unarmed attack but you must be doing lethal damage) or blood drain in proportion to the creature's size when determining how much blood was consumed from a living target. If you have both drained and bitten, use the greater percentage. For example, if a vampire with a bit attack drained 1/2 of a large creature's constitution dealing about 1/4 of the creature's health with blood drain and dealt about 1/2 of the creature's health from bite attacks, it would count as having consumed a medium and a small creature.