Some sketches from last Tuesday's life drawing class:
I always prefer starting the day with short poses. I find spending the entire morning on several 10-15 minute poses is a good way to "warm up" and get into the swing of things.
Regardless of how long a pose is held, there's always a danger of becoming complacent and not looking at the pose enough when drawing it - i.e. you've really got to see what's happening with the body. The less time you have to spend on the drawing, the easier it is to identify the most important, essential elements of the pose. Sometimes all you need to get correct is the twist in the spine or the tilt in the head or just the placement and angle of the feet...any of these on their own can be enough to describe the action.