Is half an hour long enough of a lesson time for beginners?
Yes. In the beginning the half hour gives the student a lot to practice at home. They could mentally cope with a longer lesson, but playing an instrument also involves a physical skill. To get the physical side down takes spaced repetition. In a half hour they will get enough material to be able to learn well and develop proper technique. As they progress, and at the advice of the teacher, you can go to a longer lesson time.
