Until it's actually time to die. I bet a lot of people change their pro-euthanasia views about that time.
And you are very correct...at least one person died because she changed her mind at the last minute but the hemlock support volunteer jumped in and held the plastic bag over the person's head, killing the person.
These and more stories are in the books by Wesley Smith, Culture of Death and Forced Exit.
Additionally there are studies showing that disabled people who had said they would not want to live in certain disabled ways, change their minds when it happens to them and enjoy life.