Mom is 90, early-to-mid stage, with hyperlipidemia, hypertension, diabetes type 2 and Graves disease. I know life expectancy depends on so many factors, and the progress of the dementia varies from person to person. Just trying to wrap my head around what to expect and how soon.
My suggestion as was said before, enjoy the time you have. Make memories anytime you can. Never say no. Anything you said no to before, make a point to do it. Enjoy the stories and memories while they are still there. Make a point to take pictures to remember memories as they are loved by my grandma so will probably be loved by your mom. Take time to spend with her when you can. Make random phone calls if you are far away, anything you can do to spend time with her. Hopefully things progess slow but only time will tell.
My mother was only 76 and had been in a nursing home for a year when she suddenly collapsed and died - congestive heart failure was the main cause of death. She had actually been doing relatively well - getting out and enjoying activities right up to the day before she died - and enjoyed her new roommate quite a bit. But we just never know. It was a shock to us, because we thought we had at least another year...maybe two or three years....but that wasn't in the master plan for her, apparently. It was very traumatic for us, especially with how it happened - but we have to be thankful that she didn't have a stroke or something similar happen and linger in a life she could no longer enjoy. She would have hated that. I prefer to think she went quickly and without pain or knowledge that she was going. It seems that's just how quick it was. Sitting up on the side of her bed one minute, and gone the next.
As I mentioned...just enjoy your mom now. She will value that far more than anything else. And if she has memories and stories that you've heard a hundred times, listen to them just once more. And write them down - later, you will be very glad you did.
How well are the conditions being managed? Having untreated hypertension has a very different outcome than blood pressure that is kept in check. Same with diabetes. Her doctors may be able to give you some longevity prognosis, but it would be only a general range.
However, given her age and medical conditions you've listed, I'd definitely say you need to make sure you are prepared for the fact that something could happen very suddenly and she could be gone without warning. Not trying to be harsh or morbid here, but your mom has advanced age and medical factors at play here.
I wish we could give you a more definitive answer, but no one will be able to do that - not even her doctors. Just enjoy the time you still have with her, while you can. Spend time with her, talk to her, read to her if she likes that, show her photos of family - that kind of thing. That's the one thing that you won't regret - that you gave her your time when it meant the most.