This is certainly not obvious how to configure so someone ought to write it up on docs.moodle.org (hint, hint), once they have checked the steps in detail.
Roughly what you need to do is:
1. Create a new role called something like 'Needs more time'.
2. Configure this role so that it has the single capability 'mod/quiz:ignoretimelimits' (or whatever it is called.
3. Assign this new role to the student in an appropriage
context. You could assign it at the site context, so that they can ignore time limits in all quizzes in all courses, or you could just assign it on a single course or quiz.
There is no way to give some multiple of the time limit, like two or 1.5, you can only ignore it altogether (but you could enforce the time limit in other ways, for example if this was an invigilated exam.