On the WTM boards, a poster has a young high school level DS who is on track to be on the IMO squad for NZ. I think he spends 3-4 hrs a day on math. This is less extreme than it appears, since as a homeschooler his regular math is done through AoPS books and classes. So that is similar to the 1hr/day class time and 1hr/day homework time for a typical public school kid. Net of that, it leaves ~2hrs per day for more math... either other classes and subjects or explicit contest prep.
The WTM accelerated board has a couple of folks who have kids who have made it to the AMO/Mo(S)P level...
I think the total commitment is going to depend on how useful your school math classes are.
