How long is it supposed to it take for a seniors vision to change?

Depends: Most people will notice changes in vision with age. All of us will eventually develop cataracts, which can fortunately be removed. Some will develop a disease called macular degeneration, which can result in severe vision loss. Even those who don't will tend to notice they need more light and maybe some magnification to see clearly. That is due to a decrease in contrast sensitivity with age.