In my experience good models can hold really still for about thirty minutes - sometimes longer if the pose isn't too stressful. At some point models and artists need to take breaks. Frequent breaks are best so body parts don't get stressed. Returning to pose things are almost never the same. This is particularly pronounced in multi-day sittings. During a live session you have to be willing to make adjustments in real time.
|Nude Study, (day one), oil on canvas, 11" X 10", 2013|
For the above painting I was setting up my pallet when the model got in pose for a second sitting. She captured the original pose but it was obvious her head and top hand were different. She looked very natural and comfortable so rather than try to recreate the original pose I decided to paint it the new way. I can't really say the second pose is better but I like the natural slump of her chin on her hand.
Usually pose shifts aren't so dramatic but sometimes even subtle differences can necessitate making significant changes.