Hello
@Yvonne7035 and welcome to Talking Point.
Unfortunately there is no knowing how long different stages will last, some people progress quickly, some more slowly.
I asked the doctor exactly the same question when mum was diagnosed and he shrugged and said how long is a piece of string? Mum was progressing very quickly at that stage and I feared that she would not see the end of the year, but then is slowed down and she lived for another 3 years before being suddenly propelled into the last satges.
With dementia you just never know what is round the corner, so you have to be flexible and react to things as they occur. It doesnt sit well with me as I like to be organised and sort things out in advance, but I just cant do that. Some things that I worried about (like her not recognising me) never happened and other things that I never considered (like the way she thought she was married to the vicar) became a big problem. It kept me on my toes.
The only thing I would recommend that you get organised in advance is legal stuff like POA (finances and Health & Wefare) for your dad and getting him to make his will. By the time you need these it is too late!