In Bulgaria they are for four hours a day (for skiing). They usually start at 10:00 for around two hours, a break for lunch, then another 2 hour session in the afternoon finishing for about 15:30.
This is how it was for the two times, I have visited Bulgaria to ski.
That seems to be about right. They quote four hours of lessons per day - starting at 10am, ending when ya too shattered to face another run! This often leads to nearly 5 hours per day!!
Remember to tip ya ski instructor if he was good!!