You try ingesting a mystery poison and resigning yourself to the gruesome fate of a lake full of zombies and see how long you feel like sleeping. (Do not try it. Regulus wouldn't wish that on anyone. He just hopes Kreacher can forgive him for not knowing what he was being volunteered for.)

It's a good few minutes before he makes it outside, but he does get there, steady enough on his own feet that he's not leaning on anything.