I hate to say it depends but it does. If we're really in a rush and I have good content made by the 3D team that I can paint over, then I can get it done in 6 hours or less. If I'm doing everything from scratch (ie hand drawing the background and the character from approved Lucas reference), it can take me 8 to 16 hours depending on how much I have to do. The initial iteration phase can go pretty quickly, if we include Lucas in that phase it can take a week or two before I can get started. Iteration and feedback can add anywhere from 2 to 16 actual work hours on top of the initial work. It all depends on Lucas approval and internal review.
But if we're really really short on time and need to get something in ASAP then we've been able to turn these artworks around in 2 days from initial concept sketch to final illustration approved by Lucas.