Much of the time is in the "details".
If by "like Kahn academy" you mean "a website that can show videos, possibly search then and organized into courses, in modern browsers with 100 visits a day", that's something an experienced person could do in some weeks, I think. Like a proof of concept.
If instead you mean "how long would it have taken one person to make all the features of Kahn academy, just as polished and stable, with quizes and statistics and teacher mode and ..., all with many thousands of visits", then you're suddenly measuring in years.
So really, it's hard to tell...
Mark