It's anyone's guess...each person is different. The AVERAGE life expectancy of someone diagnosed with AD is 8 years from diagnosis. Many live much longer (close to 20 years) and others (usually those who are diagnosed in the 40's, 50's) can live only 3-6 years from diagnosis. May factors come into play..age at diagnosis, if other close relatives have had or have AD, how quickly the person is declining, type of care received, etc.
edited to add: the social security actuarial tables show that a woman who is currently 83 has an average of 7.97 years to live.
My husband was given the DX in 2009 and died 11 Aug 2013 but I was suspicious a couple of years prior. He did not die of ALZ, it was a cardiac arrest having to do with severe vascular disease in his legs, arms, heart...
My husband was 61 when he received his official diagnosis from an esteemed (and well paid) specialist. He said DH's life expectancy from this disease was 2-20 years. It doesn't take a mental giant to figure out that is a good guess-timate for any 61 year old.
When I asked Sue's PCP the question about life expectancy, when she was DX'ed in 06 his reply was the best yet. "I'm a lousy fortune teller" When she had surgery for bowel cancer '09, she was told by both the surgeon and oncologist that without chemo (which she declined) she had less than 6 mos. to live. She went nearly 3 years post op symptom free of cancer, AD continued to advance unimpeded. Which killed her??? both are listed as cause of death or her Death Certif.
6 and a half years from diagnosis, although I believe Dx should legitimately occurred 2 years earlier, so we were about right on the average. The dementia caused it, nothing else.