Does Jimmie Johnson have a plan for when to hang it up?

On Friday at Dover International Speedway, Jimmie Johnson tackled a tough subject: Retirement.

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This weekend at Dover International Speedway, Jimmie Johnson is slated to make his milestone 500th career Sprint Cup Series start. His 15-year career has seen him go from a relatively unknown to one of the sport’s most successful and accomplished drivers in history.

Since making his Sprint Cup debut in 2001, Johnson has earned six championships, 74 victories (eighth all time), 204 top fives, 310 top 10s and 33 poles. Sitting fifth in the Chase standings with four victories on the year, Johnson shows no signs of slowing.

"My head is still spinning," Johnson said Friday at Dover, where he has a record 10 wins. "It’s been a life-changing experience, one that I’m extremely grateful for and an experience I think really shows what the power of people can do. The situation that was created between Lowe’s and Hendrick Motorsports, starting the team, Chad Knaus (crew chief) and myself being put together, the guys that have put their heart and soul into this team and started it and created it. The people made the success of the No. 48, all of us together, the team. To be on that team is just something that I dreamed of, but I still couldn’t have dreamed this big and have this much take place for us."

To drive home that point, Johnson recently signed a contract extension with Hendrick Motorsports to remain the driver of the No. 48 Chevrolet for two more years.

Although Johnson, 40, sees some of his fellow competitors making plans to hang it up, the six-time champion is not ready to cross that bridge just yet. Teammate and mentor Jeff Gordon is stepping away at the end of the 2015 season, while three-time champion Tony Stewart announced earlier in the week he would retire from the Sprint Cup Series after the 2016 season.

"I think for myself personally there is a moment when it feels like work, there is a moment when you don’t want to get up and go to the track for whatever the reason is. That is what every driver or any athlete . . . is trying to be aware of and step down at the right point in time," Johnson said. "We are very fortunate as race-car drivers that our career can go much further, but I have to say that the Mark Martin-racing-into-your-50s-era is probably gone. The schedule, the demands, everything that goes on, I would say mid-40s is probably going to be that point for a lot of guys, and we have a lot of guys getting close to that age now."

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Watching how other veteran drivers have handled their retirements, Johnson is not ready to set a date for the end of his career. Instead, he says he will know when the time comes.

"I do not have a number and I have not picked a number. As a kid growing up racing and as I got in the sport, I didn’t say, ‘When I get to this age, I am going to step down,’ " said Johnson. "I haven’t had that conversation and have not picked a number with my wife and said, ‘OK honey, this is the point that I am going to stop.’ It’s really been based on feel, and I have (wife) Chani’s support on that as well. When I feel like it’s time, I am going to make that decision. Certainly don’t feel like it’s time now.

"I remember watching Rusty (Wallace) pick a number and then remember talking to Rusty in years following that, and I still think he’s mad he stopped. I still think he feels like he could be out here racing with us and winning races," Johnson said. "So conversations with him, with Dale Jarrett, with other guys . . . I have always been curious. Why, when, what tells you to stop? Mark (Martin) tried a half dozen times to retire and couldn’t walk away. So I want to make sure I do it once and (don’t) keep coming back. What I am looking for is that moment. That moment that you say, ‘All right, it’s time.’ When that shows up, then I will step down."