Here are two ends of a continuous spectrum, model-wise... one fidgety and unable to maintain still for the duration of her poses, and the other a virtual statue.
Both challenged my rendering skills in differing ways.
I'm quite grateful for them both.
The latter, however, really gave me a chance to push myself in ways that I am rarely able to do within short pose confines. Lean, athletic, and with tremendous muscle control, she was able to strike and hold very difficult poses with almost mechanical precision... while still maintaining a supple, almost fluid gesture in the positions she held. Very comfortable in her skin, this gal...
The gamut runs from 1 minute to 15 minute sketches for both models... it is relatively easy to identify which drawing is what time limit. Make a game of it if you're bored;-)