This depends on so many factors that it's really hard to pin it down. Could take a day. Could take a week. I work a full time job in addition to making art, so I usually only have evenings and weekends to get work done.
What materials do you use?
Various weights of watercolor paper
Matboard (for lifting layers)
Do you take on commission work?
Usually yes, but I am way behind on projects. So at this time I am not taking on commission work. This may change in the future.