Having been a university lecturer (aerospace engineering, not recently apart from a few days specialist contract teaching each year), my experience was that 15 hrs/week was considered an absolute maximum for anybody with a proper job (that is with management and research obligations as well), and that would be pretty gruelling. People who did nothing but teach, maybe 20-24, but they could only do that for a few years, before their knowledge base became stale and in many cases out of date.
That's all within a true 40-50 hour working week.
I can't see that TK instruction should be regarded as much different?
G