The club I belong to has a minimum of 3 years but once you attain the last belt before black you must train a further year before being put forward for black. I attained mine after 5 years but at my wish as i felt i wasnt ready after 4. Did me the world of good as i now have my 2nd dan and am working towards my 3rd we have had some kids as young as 9 attain their black belts but in their defence they started at the age of 5 and they did have a natural flow for the style... like everyone else has said every school has its own standard. its just a case of picking a good club that your son enjoys...
