(Followup to post above)
OK, so I survived. Actually it went very well. The model held her pose for 20min. blocks and took a 10 min. break returning to the exact same pose. That’s how the whole session went for three hours. I did three sketches blocking in form and checking angles. Then I selected the one I felt most confident with and started to build on that one with tone. I think that was a good approach to the 3 hr duration. Another way would be to select certain areas to focus on in detail like legs, hands, portrait, etc.
Most in attendance drew with graphite or charcoal. A few added to their sketches with pastels for tone. One person used acrylic paint and another did watercolor washes.