A pint.
About 568 milliliters.
One-ninth of an adult human's total blood volume. Theoretically, losing that much blood would cause mild or early-stage shock symptoms—palpitations, shortness of breath, dry mouth, weakness, and so on. To a wizard, though, it barely counted as an injury. On the contrary, many old-school wizards believed that moderate bloodletting had certain health benefits.
Thus, upon hearing Dr. Duzem's plan, Professor Monteria's attitude quietly shifted. After weighing the matter, he slowly yielded in his heart and instinctively began considering the feasibility of carrying it out.
"We can't make a move at the school." The professor spoke softly, his face still hidden in the shadow of his hood, as though that made him feel more at ease. "Too many eyes are watching her there. Stirring up trouble on our own will do our experiment no good."
"But when she's an adult, she's extremely vigilant and almost never leaves the school alone."
Dr. Duzem slowly paced to the bookcase, dug out his two eyeballs, and tossed them into a glass filled with medicinal solution to wash them. As he did so, he analyzed what he had observed earlier. "...And when she slips out of the school alone, she's usually in the form of a child, making it impossible to safely draw a pint of blood... So Beta Town isn't an appropriate choice either."
Clearly, he had always kept a close eye on the Philosopher's Stone he had "lost."
Professor Monteria gave the study's owner a long look, his gaze sweeping over the tall wardrobe mirror in the corner. He murmured, "If you'd been lost in a mirror for decades, then wandered the Illusory Dream Realm for decades more, you'd become vigilant too... Since neither the school nor Beta Town will work, we'll wait until she leaves both places."