The answer has varied. In 1991, after narrowly missing a world championship medal, he vowed to quit immediately; following last year's Olympic 100 metres triumph, the revised estimate is 1994. But Christie's response to the inevitable supplementary question of who will follow him has remained constant. That runner, he says, is Darren Campbell.
At 19, this rangy, amiable six- footer from Manchester has achieved far more than the late- developing Christie had at the same age. Having taken English schools and AAA Under-20 titles, Campbell won gold at 100 and 200m in the the European junior championships of 1991, adding a silver in the relay. Last year, he won silver in the world junior 100 and 200m, and gold in the relay.
Now it is as true in athletics as in any other sport that top juniors do not necessarily make top seniors. The experience of the Edinburgh sprinter, Jamie Henderson, whose career never progressed significantly following his European junior 100m win in 1987, stands as a salutary reminder to Campbell of what damage injury can wreak. And there have been others who have failed to achieve their potential after even swifter starts to their sprinting careers, such as Ade Mafe, an Olympic 200m finalist in 1984 at the age of 18.
So what makes Christie - and many other expert observers - so excited about this runner's prospects? 'He's got the right mental attitude,' says Christie, who has roomed with Campbell before European meetings such as last year's Oslo grand prix. 'We get on really great. One of the things about Darren I find different from other juniors is that he gets in and mixes. He doesn't stand in awe of anyone.
'His problem is going to be surviving the next two years as a senior. But in a couple more years from now he will be quite a guy. He can be the one to pick up where I leave off. As long as he wants it enough.'
Campbell admits that there was a time when his commitment was questionable. 'I might as well be honest,' he said. 'After winning the European juniors the success got to me. I felt I didn't need to train as hard.'
It was not an attitude which thrilled his coach of the time, Earl Tulloch. 'I spoke the truth to him,' Tulloch said. 'He didn't like it. He's got a tremendous amount of ability but if he doesn't watch out he could end up wasting it.'
By the time Campbell had sorted himself out and linked up with other young international sprinters such as Katharine Merry and Steven Gookey in Keith Antoine's training group at Birmingham, his 1991-92 winter had been severely disrupted.
He narrowly missed selection for the Olympics, and was only partially satisfied with his achievement in winning world junior silver medals behind Ato Boldon, of Trinidad. 'Deep down inside I feel if I'd done a proper winter I could have won. Getting a gold medal in the relay was a relief for me. I don't think I could have handled coming back with three silvers.'
Campbell, who is training as a fitness instructor at a health club in Manchester, feels nevertheless that the experience of the last year has been vital. 'It made me realise that I have got to work hard all the time and keep my feet on the ground,' he said. 'This winter I've worked really hard for the first time. Our group has just spent a week doing hill-work on the dunes at Bournemouth. If that doesn't pay back, I don't know what will.'
Antoine has worked hard to concentrate his young charge's mind on winning a place in this year's World Championships. 'We are building towards him emulating Christie and coming back from the Atlanta Olympics with a substantial piece of hardware,' he said. 'It is a huge responsibility inasmuch as you know the guy is destined to run very, very quickly and you have to get him there.'
Technical guidance is not something which Campbell, who runs with the grace of Lewis rather than the power of Christie, needs to spend much time on. 'He runs so naturally, as though it was what he was born to do,' Antoine adds.
Had Mrs Campbell not put her foot down when Wigan Athletic wanted to sign her son on schoolboy terms, Campbell's talents might now be employed in a different sporting arena. He was disappointed at the time. Not now.
'I didn't have a picture of myself as a soccer player, not like I have in athletics,' he said. What is that picture? 'Olympic champion. And Olympic 200m record holder.'
A time of 19.73sec is some target for a runner whose legal best stands at 20.87. His 100m best of 10.37 will also need considerable revision before he can approach the crucial level inside 10 seconds. But if confidence has anything to do with it, those times will fall.
Christie has always said, half jestingly, that he will get out before Campbell starts beating him. Clearly he does not expect that to happen in the next two years. 'We'll have to wait and see,' Campbell said. 'I see it as an honour rather than a burden to be seen as the man to follow Linford. But if I do beat him it won't be the be all and end all. That is not the only thing I want to do.'
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