Long non-answer:
I don’t think you can measure the requirements for progress in kilometres. For example, it can make a world of difference whether you cover the same distance by running 10 metres daily for 1000 days, or 1 kilometre daily for 10 days. Or half a kilometre a day two days a week for 10 weeks.
If you follow a schedule, then an experienced trainer might be able to tell you for how long you’ll need to follow it before you can expect to have doubled your stamina.
And as you say, normal people don’t exist. Different people will have a different metabolism and cardiovascular system and whatnot. A doctor might be able to tell you what effects yours will have on your development, and may even be able to estimate how much it will affect your progress.
But I’d say if you really want to know, there’s only one feasible way to find out: work out, and take notes. Then by the time you can run twice as long before having to stop, look back and see how long it took you. :)