Language changes all the time, most often in usage but also in spelling and grammatical form.
At what point does a widespread misspelling or incorrect grammatical usage become acceptable and correct?
There is wide disagreement as to what "acceptable" and "correct" mean. One measure is whether a definition or usage shows up in the OED. Up until recently, revisions in the OED were exceedingly slow and so lagged far behind popular usage. Toward the other end of the spectrum is the pure "ghits" approach, which figures that as soon as the number of Google hits reaches some number (or in the case of an alternate spelling for example, when the ghits reach parity with the earlier spelling) it would be considered acceptable. Some usages become "acceptable" in casual conversation quite quickly, but decades or centuries might pass before it attains "acceptability" in more formal contexts (say, a PhD dissertation). I believe that with the increase in global communication and technological advances, the pace of invention of new words and usages is accelerating. Thus the canons and the self-appointed arbiters of "acceptable" use need to change more quickly as well. There will always need to be a certain waiting period to see whether a new coinage "sticks" or is just a brief, fleeting fad. Again, what is an appropriate period is a matter of opinion.