The events that feature Merlin and the Holy Grail occur 1000 years before Arthur becomes king in "The Broken Kingdom."
The rest of the events occur 500 years after Merlin drinks from the Holy Grail, right before the quick-scene in "Dreamcatcher," and 500 years before Arthur becomes king in "The Broken Kingdom."
They also occur centuries before "The Apprentice."
The original airing said that the rest of the Camelot flashbacks take place 200 years before Emma comes to Camelot. However, Adam Horowitz confirmed it was a mistake.
