I would say shortly because more a horse is younger it has concentration. I think 5 minutes of work is good when they are between 2 and 4 years old. After their 5 years it is rather towards 10 minutes. Be careful, it's not 5 minutes in a day, but you have to take breaks after 5 minutes (or 10 minutes) of work for the young horse to rest and think of something else. Once he is a little stretched his legs and he has eaten a blade of grass, you can start again asking him to be concentrated.
I would say it isn't too high! the average horse can concentrate for about an hour before they start switching off so younger horses probably switch off sooner. Carl Hester (british dressage rider) trains his young horses in a concentrated schooling session for between 15-30 mins a day