How can I give a student extra time on a Quiz in Moodle?
Adding extra time to a Quiz for a particular student is easy.
While editing your Quiz, find the Settings Block. Under "Quiz administration," select "User overrides".
Select the "Add user override" button. Select the student you would like to set overrides for and then set the overrides. Choose "Save".
(Source: http://docs.moodle.org/23/en/Quiz_FAQ#How_can_I_give_particular_students_extra_time_in_a_timed_quiz.3F, Last Accessed May 16, 2013)