Downton Abbey could run for 10 years, according to its producer Gareth Neame.
He told the New York Daily News that he doesn't want to see it end any time soon, but that he will only keep it going as long as it stays on top of its game.
Gareth said: "I think it is going to go on for a while. Right now the show is still growing in the US and it would be awful to think of the show ending.
"I would rather let the show run between four and 10 years, I imagine.
"I would rather that we picked the right year [to end] and that in 20 years' time the show was loved, rather than we went on a season too long and people fell out of love with it."
Downton fans have been concerned by rumours that creator Julian Fellowes would quit the show to work on his new NBC New York period drama The Gilded Age.
Gareth said: "I dread that day when there might be a time we might bring a couple of other writers into that process.
"We have been very fortunate and a key part of the show's success is Julian. He invented all the characters and every word they say and that has got to be part of the success of the show.
"I hope to keep him for as long as we possibly can. What Julian does is unique."
Downton Abbey's fourth series is currently in production and the new stars joining the cast include EastEnders' Nigel Harman, Death In Paradise's Gary Carr and World Without End's Tom Cullen.