Some dementia progresses slowly over years. And they can continue to live a fairly normal life for a long time. Some advances very quickly once it starts. A long time friend began showing signs of dementia a year ago. It soon developed into full blown Alzheimer's. A month ago his wife had to move him to a memory care facility as she could no longer handle him.
Just be aware that you could experience either extreme, or something in the middle. Do live life to the fullest as long as you can and accept whatever life hands you.