Each session can be from 40 minutes to all day.
Audience size can be one to three classes but fewer is always better as it gives more time for hands on.
A typical Primary day timetable is something like:
9:15 to 10:00
10:00 to 10:45
11:00 to 11:45
1:30 to 2:15
2:15 to 3:00.
Often it is one class in the morning and one in the afternoon or three classes spread over the day.
A morning only visit can be one class all morning, two classes having half of the morning each or three classes having a third each.
A typical Secondary school visit involves six sessions of 40 minutes.
As you can tell, the timetable is up to you,
I will arrive at your school at around 8:20am.