Students should be able to do quizzes with a time limit whenever they choose.
I run a course where we use quizzes with time limits to test students. They like it a lot. The only complaint they have is that quizzes must be timed to open at a certain hour. It would add a lot of flexibility if a student could do a test with a time limit whenever he chooses. With such a feature, student could do everything according to their own timetables, without the need to be online at, let say, 1800 hours on Monday etc.