Why the witches always give a deadline

In movies, when bargains are being made with witches to change something about the future, the witches always give some sort of deadline. I’m thinking about witches like the Little Mermaid, or Brave.

The witches are playing guides to the main character, and guides always call those they are guiding to action. Without creating a deadline, there is no immediacy. The call to action is weak.

These guides may not always have the best intentions, like Ursula, but they do get the characters to take action either way.

A ticking clock is important for getting people to take action.