Allen's Monster (Or "Alien's Monster" if you warm limeaded)

"Promise me you'll stay hidden in here, Grover." "I promise," the creature replied. "Good," exhaled Allen, "So I can finally go out in public." Allen stood up, stumbled from the unforeseen dizziness, and shambled through the door into the blinding light outside. His silhouette faded away into the piercing white. "Terrific," the monster snarled, "now I'm left to wonder. Left to wonder without a damn thing to eat." Hours passed and Allen had not yet returned. The creature's beady eyes drooped like wilting flowers, exhausted from the lack of sleep. Contrary to what Allen believed, the monster was quite loyal, and rarely considered abandoning the entire effort. He jolted upright and blurted "No! I need to be awake when he gets back!" But the room remained vacant of human life. The sun was beginning to set now, and Grover was trembling with fear. Astoundingly, he had never been left alone for so long, but the feeling was familiar. The cold air, the gaping loneliness, somehow this experience wasn't new. Suddenly he reach a revelation. "How long have I been alive?" he wondered aloud. His words retreated to every corner of the room and faded away unanswered. His mouth remained agape, as if expecting a voice to answer his query, but no such interjection occurred. But hearing the question aloud got his mind stirring. How long has he existed?