Hi Zoe, welcome to TP
As Tony says there's no fixed timescale everyone is different, it's more a matter of dealing with what's happening than trying to predict what will happen.
You should contact Social Services and get them to do an assessment of his needs, they will come and talk to him and tell him what help and benefits are available to him. It also gets him on their radar for the future which may come in useful later on, you can self-refer for this it doesn't need to be done by the GP. Google the name of his local Authority and "Vulnerable Adult Assessment" that should do it and you can probably do it on line, they are required by law to do an assessment if you ask.
As he has a diagnosis then in some respects it stops to be a medical problem so you'll get more help from Social Services, once diagnosed and put on the appropriate medication that's all a GP's can do.
The figure of 8 years as a expectancy is based on an average aged diagnosis, anyone diagnoses under 65 is classed as "Early Onset" so I wouldn't put too much faith in it in your dad's case.
"Words of wisdom" Don't volunteer for anything, you're a mother to a 1 year old child and have a job too, think of you and your family first and don't let guilt or the Social Services push you to the limit, they seem happy to sit back and let you do it all, don't be morally blackmailed into doing anything you don't want to. A bit harsh maybe he's paid taxes all his life so it's time to get some back, nobody has children to "grow their own" carers it is the states responsibility, do what you can but do what you want too.
K