There are 4 exam periods for my school on Wednesday, as we have two computer rooms. Two groups of 30 people do an exam at once in these two rooms.
8:50 am-10:10 am
10:30 am-11:50 am
11:50 am-1:10 pm
1:50 pm-3:10 pm
So, no, there aren't any time frames. I can leave as soon as I finish my exam...