It would be useful to be able to specify extra time either as a set number of minutes or as a percentage of the regular quiz duration. This would have to be on a per-user basis (or possibly per-user, per-quiz) as not everyone would need extra time.
Somehow a user could be flagged as "extra time" in their main profile, and each quiz has a "extra minutes" or "extra %age" setting when you create the course, so any user flagged as needing extra time would have the quiz duration extended automatically.
I work at a college and we have two settings depending on the special need involved of either 33% or 50% extra time. this may be easier than a typed-in minute or %age duration...?