The actor, who plays valet John Bates, joked that the hit period drama would turn into Emmerdale if it continued into a more recent era.
"I can pretty much say all of us know when Downton is going to end," Coyle told the Radio Times.
"This is a show with a finite life. If we bring this into the '50s, it's Emmerdale."
Speaking of his own character, who is accused of his wife's murder, he added: "Bates was in the Boer War. He would have killed a lot of people. Does that mean he can kill his wife? What does it do to you?"
Insiders have predicted that Downton Abbey will run for two more series, before drawing to a close when key actors move on to other projects.
Jessica Brown Findlay, who played Lady Sybil, is already filming Winter's Tale with Russell Crowe, while Michelle Dockery and Rob James-Collier apparently want to launch US careers.
Downton Abbey creator Julian Fellowes recently revealed that he is interested in creating a prequel to the series, focusing on the beginning of the relationship between Cora and Robert.
