One of my freshman professors accidentally did nearly that. The final exam was 3 hours. This was normal at my school although many students finish in 1-2 house. After realizing nobody was close to finishing after 2 hours and he had greatly underestimated the difficulty, he expanded the time limit to 6 hours!
I will say it's not practical to have exams that long. In this case, the dorm required me to move out immediately after the exam and my parents were waiting to pick me up, so I decided to leave after 4 hours to avoid unnecessary panic or having to drive overnight. In hindsight, the professor probably would have let me make a phone call, but that didn't occur to me at the time.
I will say it's not practical to have exams that long. In this case, the dorm required me to move out immediately after the exam and my parents were waiting to pick me up, so I decided to leave after 4 hours to avoid unnecessary panic or having to drive overnight. In hindsight, the professor probably would have let me make a phone call, but that didn't occur to me at the time.