He can't literally do it forever. He's only got so much room. But he keeps drinking for many minutes, until unregenerating prey would have been exsanguinated a dozen times over, until it hurts to swallow more and then some, until even hyperefficient vampire metabolism can process nothing more and is reduced to storage, distending his stomach.

Finally he is as sated as he physically can be, and he's just licking venom into the wounds, making small sounds in the back of his throat. When he looks up, his eyes are brilliantly burgundy.

"You're a marvel," he gasps, having neglected to play at breathing throughout his meal.