It's pretty complicated stuff, but I actually think that most people are capable of it, if sufficiently motivated. This assumes you're *capable* of learning Integral Calculus (and all the topics that precede it). But later on, I suspect it will become really difficult to get through the exercises with just Khan. If you're an average student who's just really determined to do it? Maybe at first it's 20 minutes for each video (watching them twice), 30 problems for each skill and 3 minutes per problem. An hour a day, 180 days a year, for a little more than four years - basically, as all your friends are finishing high school. So let's say the average time is 20 minutes of video, and 100 minutes of exercise work, times 390, which is is 46,800, or 780 hours. By the time you finish, you might be watching 30 minutes of video and doing 30 problems that take 5 minutes each (and I'm unconvinced that Khan Academy is going to be able to *educate* anyone - I think you'll still need to find other resources to fill in the blanks that the videos and exercises leave in your understanding - this is why you'll always want to use Khan *and* an actual teacher). Maybe it would take 10 minutes for the video, 15 problems for each skill, and 1 minutes per problem in Algebra - but it's going to slow down a lot as you get to Trigonometry and Calculus.

If you're a brilliant elementary student who has just finished all the K-8 work but hasn't learned anything more yet, then you'll need to watch videos for each topic and work on the exercises. Averaging two questions per topic and taking one minute per question, that's 780 minutes and you're done in 13 hours. If you're a fantastic math PhD, then it's just a matter of answering one or two questions for each one (I think Khan gives you more credit towards mastery the faster you answer questions, at least at first). So let's assume we're talking about the remaining 390 exercises, Algebra through Integral Calculus.

Right now (Jan 1, 2015), there are 926 math exercises on Khan and 536 of them are for grades K-8.

Khan academy math algebra 2