Time limits on individual question within a quiz is not a feature that Moodle currently supports. Quite a nice idea though.
This is possible with the Active Quiz plugin, although it is meant for real-time quizzing (and intended for mobile device use as the input method). I would love to the feature of time limits per question to be supported though-more options and flexibility is never all that bad. Since Moodle already supports time limits for quizzes, I can't imagine it would be too difficult to integrate on a question level.