How quickly does dementia progress?

My 90-yr old father was in independent living up until October. Starting to observe some behavioral changes such as confusion with time/day/night and anxiety around being ready in time for appointments (would get up and get ready hours before). Starting to have accidents during urination/unstable gait and fall risk. Cognitive assessment resulted in onset of dementia. Moved dad into assisted living and he is like another person. He sleeps all day, he does not remember or acknowledge when family visits - and when he does, he wants them to leave, he isn't nice, he even looks different. He now has sun downers episodes, he cries at night, he says he is unworthy. My question, how quickly does one go from "onset" to "full blown" dementia. How long can someone live like this, what is the life expectancy? I know you all have your own unique situations and I'm interested to know your stories of progression of these symptoms. I'm starting to grieve the loss of my father. Thanks for your time.