There's a saying about Alzheimer's: If you've seen one case of it, you've seen...one case of it. The disease manifests in different ways and progresses at a different pace in each individual. So there's no rule of thumb for how long each Alzheimer's stage lasts. Some people decline rapidly, within months. Some level off at a particular stage for years.
Although people live an average of eight to 10 years after diagnosis, some succumb within a few years while others can live as long as 20 years.