Who or what determines when a trademark becomes a generic term?

The biggest example of a trademark losing is obviously kleenex, which is now synonymous with tissues. It doesn't matter if the tissues are made from the kleenex brand or not, they can be called a kleenex.

There are many parts of the country, mostly the south, that call all soda "coke", whether it was made by coca-cola, or not. Coke is still a trademarked name, and not a synonym for soda. At what point does coke get to keep it's trademark, and when could it lose it? Who would determine whether or not it still can keep the trademark?