You can give students extra time for quizzes ahead of the quiz being available (e.g. for students with reasonable adjustments), but you can also change the time limits for a user doing a quiz whilst the quiz is in progress.
In the same way as you would give a user override ahead of time, if you set up a user override whilst the quiz is in progress, the student's time limit will update on the next page load from Myplace.
This means that when the student moves to a new quiz page, or reaches the review page, the updated time limit will take effect.
For this reason it is good practice that your quizzes are appropriately paged, to both allow Myplace to save student answers regularly and also so any changes are picked up reguarly.
A quiz time limit is a duration limit from when the student starts the quiz attempt, it is not a limit on the cumulative time they spend doing the quiz.