How long do dementia patients usually live?

My grandpa, who is now 77 years old was first diagnosed with Alzheimer’s disease back in 2018-19. It all went downhill when he was hit with a stroke in early 2020 (and other multiple strokes later that year). All the remaining memories that he had from the present were all gone. He doesn’t recognise my grandmother, sometimes calls her mom or sister, he doesn’t recognise his children(my mom and her siblings) or me. He sometimes thinks he is in his 20s and its impossible for him to have grandkids or even kids. That was all in the beginning of his dementia back in 2020. Now he barely moves, we hired a person to help him move around the house because he’s 6 feet and none of us can pick him up. He doesn’t eat saying he’s already full even if he’s not, he hallucinates a lot. He is also a diabetic patient (since his 40s) so a nurse has to give him insulin everyday and he makes it so tough to calm him. He even resorted to violence once. After saying all this, I wonder how long he could survive and if it’s his last stage of dementia.