Grimm's law
I voted based on Grimm's Law - that some fixed percent of the language drops out or changes every 500 years (I think it is 20%).
This would (of course) have to be adjusted by major linguistic events, such as the French overlay on Anglo-Saxon, the Improved literacy of a group, or (as may be happening now) the impact of sound reproductions and transmissions such as cd's, televisions and telephones.