A year? 5 years or more? I know this was my suggestion, but I really didn't feel like there was much of a choice. Now after 7 months, I still have not adjusted to this, since month 2 I have been complaining. My health is getting worse and his siblings don't really care about her or our family. They are all about any money they may or may not get. I don't understand this, I don't care what their beef is with their mom! Yes, I am well aware that if MIL was upset with you she would make changes to her paperwork, but still...
There are some of us who can't adjust. I am one of them. The severity of the situation (the severe confusion, aggression, incontinence, etc.) is too great and the price for us (the caregivers) to pay is too high (our own failing health, no sleep, loss of friends and social life, no private time).
I agree with jeanne, if you haven't "settled in" by now, AND your health is deteriorating, I think it's time to call it quits. She will definitely progress in her dementia and you will only get worse from the stress. I would start looking for a memory care facility close to your home for MIL.
This is a sad thing for you and your husband to have to go through. As good as your intentions were, it would be in everyone's best interest to place her where she would get good care from 3 shifts of refreshed caregivers. Then you can visit her and enjoy her as your mom (MIL) and not a worn out caregiver.
What are MIL's disabilities? What kind of help does she need? Have you and your husband discussed an appropriate care center for her?
Some of us are not cut out to be a caregiver, and there isn't anything wrong with that. I never was hands-on but even running errands for my parents became nerve wracking for me. https://www.agingcare.com/articles/not-everyone-cut-out-to-be-a-caregiver-162192.htm
Just asking my boss, at the time, for time off was extremely stressful because she was like "The Devil Wears Prada".
After the first few months I noticed my health was going down hill. I didn't realize until I found this AgingCare website that I could set boundaries, and how to stop enabling my parents who continued to live in their house, with many stairs, being in their 90's. I was always saying "yes" instead of saying "sorry, I can't possibly do that".
Depending on your Mom-in-law's Alzheimer's/Dementia, it might be time to start scouting out Memory Care facilities. One positive she would be around people of her own age group, and another positive she would be around Staff who have been on this rodeo hundreds of time. And it is up to Mom-in-law to use her own funds. Let the siblings grumble, this is your health we are talking about.
Consider all your options and do what you can, at least then you know you did what you could in her best interest.