I think there is a number of teaching hours which is optimal and this varies for each teacher.
But here is what I noticed for myself. Right now I am teaching about 14 hours a week which is ridiculously low. So low in fact that i can't get in the groove. My free time distracts me from concentrating long periods of time on school. But I am NOT complaining.
On the other hand, my lessons used to suffer when I was approaching 30 contact hours a week.