It could vary with every individual soul, but I would think instantly since Alzheimer's is a physical ailment to the brian and not to the mind. You see, we (in the physical) think in terms of time as a horizontal line with a beginning and a end. Those in spirit see time as now (the present) it is like a line that completes a circle with no beginning and no end only now exists.
My grandfather passed from Alzheimers back in November 1986. Two days before he passed he came to me in a dream, not just any dream, a dream that enabled me to visit him one last time. You see, I had not been up to see him in six months and guilt was rearing it ugly head at me. He came through on the Wednesday evening and (I won't go into the details here, maybe some other time) and with a few short words, took away any guilt I had and he passed the next day. This was more than a dream, anyone who has had a visitation knows the realness, the vitality of the colours involved and the impact it leaves on the surviving one. What I can tell you is that my grandfather had his full mental compacity back and functioned normally two days prior to his physical passing. Now there are the those who will say it was only a dream... they can say what they want, it has been over 17 years and the impact of that visitation is still with me.
I realize that I run on with my answers, but I feel it is important to write about my personal experiences and what I have discovered so far.
Through countless readings with Alzheimers patients and through my own experiences I believe they shed the mental and physical limitations at the time of death or rather when they cross over since I do not believe we die.
I hope I have answered your question.