Roles and permissions

I have set a context role for a quiz for some students, overriding their normal student role. I want that role to expire in some days, so when I assign the role override I set the "Enrolment duration" to X days. This doesn't seem to work, as after X days the students still have active the role override. Am I doing something the wrong way?

I'm using a recent Moodle 1.9.9+. Thank you!