The Inspector Morse spinoff - which also stars Laurence Fox - has aired on ITV1 since 2006.
Whately told the Radio Times: "[It won't last] a lot longer. I'm now police retirement age this year, so the time is coming quite soon, I think."
He added: "You can definitely expect one more [series] next year, then after that, we'll see."
Whately - who has played Robert 'Robbie' Lewis since 1987 - admitted that he originally planned to stick with the role for just a few years.
"After three years, I thought 'I've done enough of this now, I better leave it and go on to something else'," he explained. "I said I wasn't going to do anymore, and luckily they persuaded me. 26 years later, I'm still at it."
The actor also confirmed that he had watched recent one-off prequel Endeavour, which starred Shaun Evans as a young Morse.
"I enjoyed Endeavour," he said. "I think it's got promise, so we'll see what they do with that."
It was recently reported that Endeavour could lead to a full series, with talks said to be "ongoing" between production company Mammoth Screen and ITV.
"We're thrilled with the overwhelming response," said executive producers Damien Timmer and Michele Bucker. "The cries for a series are testament to Shaun Evans's remarkable performance."
> Inspector Morse prequel Endeavour to air on Masterpiece in US
Watch a clip from Lewis below: