While this varies per-project, I ask clients to allow at least 2 business weeks to complete a project.
The bulk of the process is taken up by the sketching and development stage. Sometimes that can go quickly, sometimes it takes a bit of time. The speed of your replies to development sketches plays a big factor.
Once the sketch stage is complete, the final vector art creation usually takes no longer than 3-5 hours. I develop my concept sketches to ensure the vector art stage is more about production than exploring creative ideas.