You can use the Classlist tool under Course Admin to edit a student's accommodation setting. Here you will find a time multiplier to give more time on all quizzes in the course. 

Another option is to use the Special Access settings found under the Availability Dates & Conditions section when editing the quiz settings. Using Special Access is helpful when you only want to change the time for one quiz and not all quizzes. Special access can also be used to adjust the quiz availability and due dates for one or more students, or the number of attempts allowed.