Afterwards, students told me I was right. "It would have been possible but a tight squeeze to finish it in 60 minutes and extremely hard in 50, but with 75 we had about 10 minutes left to clean up and check our work."
I've heard that exams should be written so that students get 3-5 times as long as it took the instructor to solve the problem. I usually allow about twice as long (60 minutes for an exam I work in 30 minutes, 75 for one that takes me 35-40 minutes), and it's been a tight squeeze.
What's your experience been, as a student or as an instructor? (Please say what you took or taught.)