I would personally want to know as early as possible, so I could get my affairs in order and register my wishes around end of life care and euthanasia while I am still recognised as having full mental capacity.

It's also better for people around the Alzheimer's patient, as it will let them understand why someone's personality and behaviours may be changing, and possibly let them be bit more forgiving of such changes. It will also give family more time to plan and understand the health and community services and support are offered wherever they live.