There's often a long overlap, though. Both my grandmothers died with Alzheimers. We'lll never know how long
they knew, but in retrospect it's clear they knew of the decline for quite some time.
My mothers mother avoided hospital until it became too apparent to everyone around here that my grandfather tricker her. She'd make jokes about her failing memory for years, and while some of it might have been genuine, in retrospect odds are she noticed it was getting bad and was obscuring it with humour.
For years afterwards, she would forget conversations partway through them, but clearly be aware that something was wrong. E.g. for some time she recognised me, but would wonder when I got there and how long I'd been there, and occasionally my name and who I was would slip, but she was otherwise lucid enough to understand that this was not normal.
My dads mother managed to hide the decline until one day my grandfather was going in to hospital for a minor operation, and she refused to get out of bed. The last time I saw her before that, she seemed lucid and held a conversation. I never had another conversation with her, though she lived another decade - she went non-verbal almost overnight, but it was clear this wasn't some sudden physical change; she'd held it together until then, and gave up. It might be her cognitive decline was faster, and less cruel, but we really don't know if it was, or if she just managed to conceal it until the very brink.
How much after that she managed to hold on to enough to recognise any of us - including her husband we don't know, because shortly after she went non-verbal she mostly stopped moving.
But one of the cruelest parts of Alzheimers is what it does for those left behind - my grandfather spent a whole decade in his 70's and 80's walking to the nursing home, sitting with her all day, every day, then walking home, after she was for all intents and purposes gone.