You can give a specific student(s) extra time for an online, timed quiz in Canvas. To do this, go to the Quizzes menu area. Next, click on the quiz itself. (NOT the "edit" button.) At the far right, there will be a link called MODERATE THIS QUIZ. Click that link, and on the ensuing screen there is a list of your students. Click the "pencil" icon next to the student's name. The result will be a dialog box that has two things: one an area where you can grant extra attempts, and below that a box where you can indicate the number of extra minutes for that student. Assuming the student has not taken the quiz yet, you can ignore the first text box, but be sure to fill in the second one granting extra time.
There is additional information on this topic (with screen captures) in this Canvas help document: https://community.canvaslms.com/docs/DOC-10432 .