What can I write?
I read your posting a week ago and thought "I cant reply to that one". Now that no one has replied Im left thinking that you may think no one cares, which is certainly not the case.
To answer your question... No one will be able to tell you how much longer you will see your Dad in this way.
Please tell your Mum, and know yourself, that there is a point, which varies dramatically, when we cannot cope with our loved one any longer at home. We are not wrong, selfish uncaring etc, we just cannot manage either the physical or mental torture ourselves, and so the person with dementia is quite basically better cared for in a care home.
I'm afraid visiting Dad is not going to get any easier.
You have to decide if you should visit or not.
I have clients in care homes, where family members do not visit at all. I visit and give the family a ring.
I have other clients, who like yourself visit regualrly, and struggle to live their own lives for the pain that the visit causes, and yet they continue to visit. There is not right or wrong.
As for advice on how to cope? As long as Dad appears to be as comfortable as possible, then you can only console yourself with this fact.
Be strong, and keep in touch