My high school physics teacher did the same. He would take the tests he prepared himself to check whether the scope was appropriate for the allocated time. The factor of 3 was spot on for this kind of estimation.
I normally choose divide by 10 for exams as I could do divide by 3 for exams from a different professor/class.
I know the material cold, know the test cold, and don't have to worry about hidden traps--so I expect to take less than 1/10th the time of a student.