Some Challenges can be addressed in a day or a week, while others need a semester or even an entire school year.
If the Challenge is too big for the allotted time, the pupils will feel pressured or frustrated. If it is too small, they will lose interest.
It is important for the Challenge to be real and meaningful to the pupils. If a Challenge is contrived or something that the pupils cannot personally connect with, they will not fully engage in the process!