The hem of a true tea-length dress should hit near the bottom of the calf. That is, about 4 inches above the ankle -- give or take a few centimeters, depending on how long your legs are.
However, that being said, if you try to purchase a formal tea-length dress, most stores will either show you a cocktail dress, which may be at or slightly above the knee, or a intermission length dress, which falls between the knee and mid calf area, instead of a true tea-length dress.