I'm lazy, old, and I tire easily. 60 minutes is inefficient for me. I am definitely losing focus and not paying attention after 45 minutes, and even 45 minutes seems longer than ideal. I think 30 minutes is about right for me, but it may not be good from a teacher's point of view because it's "work" to commit to a scheduled appointment and they may not want to commit to a time slot if they can't earn more than 30 minutes' worth in the session.

I am both a teacher and a student, so can see it from both sides. As a learner, I prefer a 60 minute lesson most of the time. I am a beginner so I do everything much more slowly than an intermediate or advanced student would.  There's a lot of time spent thinking of words, correcting grammar and pronunciation,  asking questions, etc.  30 minutes would barely give me enough time to talk about what I did that day.  

As a teacher, I don't love 30 minute classes because they are over so quickly. But, in certain circumstances a quick lesson might be exactly what's needed. Personally I prefer 45 or 60 minute classes. However, I've also taught 90 or 120 minute classes as well. That seems really long, but with the right student, time just flies. 

My "sweet spot" both as a teacher and student is the 45-minute session. I also like 30-minute lessons but a full hour usually starts to feel draining to me. With people with whom I get along exceptionally well, I could talk for hours, but it is rare.