The progression rate for dementia can vary widely.
In general, Alzheimer's disease tends to develop slowly and gradually worsens over several years. It is not uncommon for people to have suffered from Alzheimer’s disease for 2-3 years before they see a specialist with concerns regarding failing memory. In most people Alzheimer’s disease takes a course of between 8– 10 years. But the disease can progress quickly in some people and slowly in others. Some people live as long as 20 years with the disease.
A small group of people have rapidly progressive dementias, typically worsening over the course of weeks to months, but sometimes up to two to three years. Fortunately, rapidly progressive dementias are rare.