Turn Into A Pumpkin Meaning

(idiomatic, colloquial) Used to indicate a curfew, or the time by which one must depart.

Example:   I'm about ready to turn into a pumpkin. You can stay up later if you want.
  Tell her I'm turning into a pumpkin if she stays out too late. She won't be getting a lift from me.
  My daughter turns into a pumpkin at 10:00, so you need to have her home before then.
  I'm turning into a pumpkin at 4:00, so we need to finish this.
  "I need to get back now!" "Why? Are you going to turn into a pumpkin?"