Many, many years ago (I think I was around 14), my Uncle George (my dads older brother but about 20 years older! I'm not sure if or how the age difference was ever explained to us) he started to become a bit confused, but it advanced rapidly over the course of just a couple of weeks.
What first happened was that he had been driving his truck for work and got lost, so he stopped to phone his son to get help with directions.
He was only a couple of miles away, in an area and on a road that he knew really well, so that seemed really odd.
Then about a week later his missus came home one afternoon to find the house all flooded - he had turned on all the taps and had gone out into the garden and started digging up all his cherished roses and other plants for no apparent reason.
He got admitted into West Hendon Hospital (now a housing and retail development) in short order, and then passed away within a couple of weeks.
My dad though, his dementia was only slight, for a lengthy period, and IIRC from the point that he first started noticing cognitive problems, through to when he finally ended up in hospital, was some 8 - 10 years.