Do most dementia patients lose the ability to speak, and though I know this varies a lot, about how long can they be silent before passing?

My mom started a low dose of Depakote on top of a medium dose Lexapro and Remeron. She had been on the latter two but really declined in mobility and speaking ability after the Depakote started. She's now in a wheelchair which is safer (less fall risk) but it is so hard to see the speech decline. All of it was happening before Depakote, but it seems to have accelerated it. At any rate, the medicine is really helping her cope better, but would like to hear how long others have cared for their loved ones after speech is gone or nearly gone. Thanks for sharing your journey.