First of all we need to get some meanings cleared out: as per the Wikipedia entry "The Eastern Orthodox Church considers itself to be both orthodox and catholic" meaning right-worshipping and universal. This is what the words catholic and orthodox mean. The question is regarding the labeling of the Eastern Church as Orthodox.
The term Orthodox Church was first used as an identifier of the Eastern Church, by Roman Catholic theologians and scholars in the years following the Great Schism, with the intention of mocking what they perceived as rigidity and extremely strict adherence to the precepts of right worship, or, orthodoxy.
However, the Eastern Church did not consider this to be pejorative at all, but rather pretty descriptive of their approach (and also pointing fingers back at the West's non-orthodoxy) so they've embraced it.
As you can probably see, it's impossible to pinpoint an exact moment in history when this identification started to take place as it was not decreed officially in any way. It just started to spread around in the years after the Schism.
PS: I can't remember where exactly I have read this so unfortunately I can't provide a proper reference, it was an Orthodox book for sure though.