The mean time from diagnosis to death in Late-Onset Dementias is 8 years but the range is of course wide and patients can survive for up to 18 years. Early onset Dementias tend to decline more rapidly.
The various Dementias decline at different rates and in different patterns
Alzheimers Dementia, being gradually progressive, has the most predictable rate of decline: roughly 3 points on the MMSE per year
Vascular Dementia, with its stepwise deterioration, may have long periods of stability and periods of acute decline which are unpredictable.