The reason why Shuangshuang said she hoped Feiyu could hatch the Illusion Butterfly Eggs by the time she returned was that this Secret Realm journey would take three years.
The rule of this Secret Realm is that once you enter, the trial lasts for three years. However, manually hatching the Illusion Butterfly Eggs only takes two years, which is why Shuangshuang mentioned it.
Upon hearing that Shuangshuang was leaving, and that she implied it would be a long time before she could return, Feiyu became anxious. "Sister Shuangshuang, where are you going? Can I go with you?"
Going to the Secret Realm wasn't something that needed to be kept a secret, so Shuangshuang directly told Feiyu. After hearing it, Feiyu understood he couldn't follow her.
