Generally a case is divided into four programs, each program is responsible for one person. For a new case, it's usually three days of sketching, two days of modeling, one day of rendering, and one day of post.
The purpose of the sketches is to completely finalize the model structure, and not to rework it when the time comes. The good sketches are the final drafts that are clearly expressed and structured, and this doesn't need to be much more than a wonderful effect.
The purpose of the sketch is to finalize the model without reworking it.