The fourth series of the fantasy drama begins this Saturday on BBC One, while a fifth run has already been commissioned.
"They've always said they would do five [years], so apparently... next year's the last one," she told Digital Spy. "[It'll be] five apparently, in total."
Coulby also revealed that she was not surprised when Merlin was renewed, despite the fourth series not having aired.
"I kind of expected it," she admitted. "I think it's been such a success that it would seem mad not to continue it out to its end."
Merlin co-creators Johnny Capps and Julian Murphy recently revealed that a film version of the hit series is also in the works.
Merlin returns to BBC One this Saturday at 7.50pm.
> Merlin: Ten teasers about the series four premiere
Watch Angel Coulby's full interview with Digital Spy below: