My DM is in the last year of this.
For us this was how it worked:
First forgetfulness for about ten years. Then there was a fall, which I think in the 1980s would have been the end of her life, because she developed pneumonia.
She was delirious in hospital for four months.
Then she came back out of hospital after four months and the delirium went away for about a year.
Then she became permenantly delirious and we started playing lots of fun games where she would tell us where she was and we would all play along. (like on the set of Downton Abbey or whatever). We read a book call "contented dementia" which was very helpful.
Then after about another couple of years, she kind of forgot how to walk entirely. The consultant in the hospital said that was likely to be it turning into parkinson's disease.
After that she went to a care home and was basically asleep for three months, except waking up for lunch. We filled in a respect form which specifies no more hospital visits and no more antibiotics. If anything else were to happen then palliative care would start, so she doesn't have any more traumatic interventions.
Now whe is awake a bit more and can chat a bit each day. I don't know what happens next.
Sorry, that all feels very personal to describe but I hope it might help you to know, because it's been quite a difficult road for us.