I get what you are saying ksefitz. I have said every word you said there many times over. In reality we have already lost our loved ones to this disease. We need peace for them and for us. There is lots of well meaning speculation in your post but the answer to your question ...When?.... nobody knows!!! The best guess can be years off. The averages or expected outcomes can be very wrong. It could be tonight and it could be years from now. This disease is a true test of perseverance and patience.
My Dad had vascular dementia. He was diagnosed in 1997 and died in 2010. Yep, 13 years diagnosed. I can tell you that Vascular Dementia is not a steady decline as is typical with Alzheimer's. Usually there is rapid decline related to a medical episode (especially heart or circulatory related) and then a period of stability. Dad had several periods of rapid decline followed by rather long periods of stabilization at the new level. Dad took his last decline in January 2009 with A-fib. He stabilized until March of 2010 when he died.
Just a note on what Doctor's tell you.... Dad's first heart attack was in 1977. He was 57. We were told he would not survive the night. Obviously they were wrong. He had another heart attack in the mid 1980's. He was not supposed to survive that one... but guess what?
He went into 5 bypass surgery with a 49% change of surviving the surgery. He did! He went on to have several heart stints, femoral artery stints, gall bladder removal, and A-fib. He was just short of his 90th birthday when he finally left us. They go when they are ready regardless of what the doctor says.
Please know that nothing Dad did caused Mom's problems. He may have covered for her but that was out of love. It is how we take care of each other. Yes, as care givers we do have a tendency to ignore our own problems for the sake of others... but those are out choices. No one can say that what Mom did caused where she is now.
What you need to do now is spend time with your Mom celebrating the life that she has lived and enjoying the moments you have left. Please do not waste this time in grief. There will be enough time for grief after she is gone. Until then hold her hand, tell her you love her, tell her about your days (even if she doesn't understand). Find the moments of you that are left for you both and make memories to carry with you. Anticipatory grief can take away the time you have left if you let it.