Reigning Formula One world champion Lewis Hamilton expects to remain in the sport until at least 2020.
Hamilton's contract with Mercedes is due to expire at the end of next season, though he is believed to be set for negotiations over a renewal in the coming weeks.
However, rule changes regarding power units will kick in from 2021 and the Briton, who claimed his fourth title this year, is unsure he will hang around to experience them.
"I think about what is ahead of me," he told Motorsport.tv show The Flying Lap.
"I have spoken to those who have retired early, those who retired late and everyone says stay in as long as you can. That's not going to happen.
"The seasons are getting longer and longer and you see the things I do outside, which take a lot of my energy and time.
"I'm enjoying what I'm doing now. I enjoy being with this team, I love racing and I still feel at the moment I'm driving at my best.
"So while the car is in this form of rules until 2021 at least, I think that's going to be the window I'm still here."
Remaining in the sport until the end of the 2020 campaign would give Hamilton a chance to match Michael Schumacher's record of seven title triumphs, but that is not a target for him.
He said: "This is crazy, people around me are now saying: 'You've got to get eight.'
"I've only just got four. It's one step at the time."