I used this with one class of 17. This amount of students fit perfectly into one day. However, I understand that not every year will be this small of a class ( I've had 32 students). This can be done over a span of two days during the last week of school.
This is a highlight of 3rd grade each year. The kids love the opportunity to take the front of the class by storm and teach the teacher something that is important to them.