Find the Classlist tool by click on the "Classlist" tab in the blue navbar on your course homepage. 

To give a student extra time on all quizzes/tests/exams made with the Quizzes Tool, click on the caret to the right of a student's name.

Click "Edit Accommodations" and enter how much extra time you want that student to receive.

Click on the blue "Save" button when finished.