Thanks for your reply. I used to only teach for 1 hour, but now that I've started 1.5-hr lessons, it seems I can't get enough done in 1 hour, but I agree that it depends on the student. (One thing that has made it really easy for me to transfer from 1-hr to 1.5-hr lessons is spending time correcting and reviewing homework in the lesson.)
I've never taught and all-day intensive lesson. I can't imagine working all day with one student! Occasionally, when a student is preparing for a presentation or interview, I'll work with him/her for 2 or 3 hours at a time. This works well as long as we have focused blocks as you mentioned- and also include a coffee break. That's important!
My private lessons are almost always 1.5 hours. Two reasons for this:
1: The student and I can cover a lot in 1.5 hours. We can start with a real Warm-Up, focus on a variety of skills and/or language points, adequately reinforce the new target language, and address any questions and small tangents that naturally pop up.
2: For both myself and the student, it makes
more sense to meet for 1.5 hours because of transportation. If it takes 30 minutes to get to the lesson, plus another 30 minutes to get back, a 90 minute lesson seems a better use of the time.