It's actually difficult to determine the frequency because we don't know how much time will elapse between the two. I also think it would be important to reassess the request, as Dr. Pageau said, when he talked about the "changing self". I don't use philosophical terms as refined as his, but he said that our self, or one's self, evolves over time. Consequently, we must let the self of persons suffering from a cognitive disorder evolve as well. We can't deny them that right.
That's why it's important to reassess requests frequently in order to ensure that patients haven't changed their mind. Some ultimately adapt to their condition or, for example, to long-term care. They may adjust to a different life and no longer want medical assistance in dying.
Consequently I can't specify an interval or number of times, but it would be important to reassess the request every time it's made to ensure that the needs have not changed.