My DM was 'lucky' (if that's the right word around dementia). She had a couple of years where she began to drift and we'd be running around propping her up. For example, she'd forget to eat. We'd discover she still had food in the freezer from the last shop.
Luckily she lived in an assisted living sheltered housing scheme so we got Wiltshire Farm Foods and one of the carers or us would come in to do a sandwich and, later in the day, to microwave her dinner.
Then she got wise to the cooking and would cook dinner at 10am - as a diabetic that led to hypos. So we moved to Wiltshire Farm Foods equivalent of Meals on Wheels. The only food in the flat then was sandwich stuff and breakfast. Her meal would come at lunch time and they'd make sure she was eating before they left (can't recommend them enough). Then the carer would do the sandwiches for the evening meal if we weren't there for the day (can't thank them enough either).
Eventually, after a major fall she was assessed in hospital and moved to a respite, then to a care home.
She liked her care home - food was excellent and, apart from incontinent knickers, you never know there was a problem.
The care home worked wonders. They even took her off of using a Zimmer frame (which respite had put her on) to using a walking stick to keep her walking. She only used the knickers for occasional urine incontinence even though the respite home said she was double incontinent. The care home doubted the double incontinence diagnosis and spent a lot of time with her paying attention to the signs and 're-potty trained her'. Apart from the odd accident (dodgy tummy) she always used the toilet right up to the end even in hospital.
I can't praise them enough as we were told at her hospital assessment that the swallowing mechanism was going. The care home always had people sitting with the residents in the dining room and noticed that she would reach for toast at the end of the meal and manage to eat it. So they gradually added 'normal' food - by the end she was enjoying unbattered fish and chips, sausage and mash and a full Christmas dinner the Christmas before she died. They thought she was misdiagnosed with a dry throat in the hospital which made it hard to swallow!
She finally died in hospital having got a chest infection that wouldn't clear. Often the way with the elderly. So I suppose old age rather than dementia got her in the end at the age of 90.
Compared to many, as I said, she was lucky. My friend's MIL ended up dying in her 'care' home, weighing next to nothing and smearing poo on the walls . Ironically hers was an expensive care home. Shocking.