I think it was Sumedho who was trusted to teach after 5years of being a monk but no mention of how many rains he had done.
there is another thread with a similar discussion on which may shed some light for you.
I know there is an ordination ceremony to become a Dhammachariya but that may just be within one monestery or sect, but I would imagine that a monk would be "given the title" when they were trusted enough to teach or started to teach as far as I can tell their is no actual formula in which the title is used.
He who knows only his own side of the case knows little of that. His reasons may be good, and no one may have been able to refute them.
But if he is equally unable to refute the reasons on the opposite side, if he does not so much as know what they are, he has no ground for preferring either opinion …
He must be able to hear them from persons who actually believe them … he must know them in their most plausible and persuasive form.