I wear glasses and am getting an eye exam today. Exactly what percentage of vision loss requires a new prescription?

It really depends on. Depends on your visual needs. It would be good to know that you are legal to drive in your state.
Seeing better. There isn't a percentage cutoff. Usually if vision is 20/40 and can be improved with a new prescription by one or more lines, it's significant. However, improvement in vision is subjective, and new glasses are good to get if you feel like they make a difference.