Actually his post above for high level players is accurate. Top tier D1 or pro potential players put in 4-6 hours/day everyday that they aren't playing tournaments from around 13-14 on and for several years. From earlier ages to 14, they are doing the 2-4 hours after school as well, with a lot of tournament play. It is sustainable for the top players, and is how they get to the top. Well managed of course is essential. Problem is some parents don't invest in the better coaches and the player does end up injured. And as tennis5 said, easier done in the good climates of CA and FLA versus indoor country.
The post above this mentioning a couple hours after school is typically appropriate for those shooting for D2, D3 or rec player.
Sounds accurate to me.
4-6 hours per day, everyday, does not sound accurate to me for any junior (or most pros) in any sport.