This video (https://youtu.be/iSV9uohbCjQ?t=258) is saying
if you're taking something like TOEFL or TOEIC that's going to be an exam, exam instills fear of one or two or three hours of you writing the test.
I understand which might mean
(That) exam instills fear for 3 hours while you are writing the test.
though I've never seen structure like "three hours of you writing" before, is it an idiomatic expression?