That's a great question. The worker bees start to eat it right away. It typically takes them about two or three days to break through and free the queen. This allows the workers to "catch" the scent of the queen and accept her before she is free to walk around. The workers, having accepted her as the queen, work to keep her fed, comfortable, and laying eggs. If she was just released into the hive without a delay of a day or two or three, she might be inclined to leave since she doesn't feel quite at home.