Julianne, everyone has a different timetable and method of working depending on style and a million other factors. An artist like Karin Wells who uses multiple glazes and paints in a very realistic and detailed style probably takes a great deal longer to complete a commission than someone whose work is very impressionistic. For example Debra Jones, who also posts here and was recognized for her talent by David Leffel in the December Artist's Magazine, can paint a portrait in a few days.
It takes me at least a month and usually more because I don't go into the studio every day; I also need time to let the oils dry and to think about what's going to happen next. Some artists have several pieces going at once; others focus on only one at a time. I would suspect that's again a matter of personal preference and style. I'm in the one-at-a-time category because I like to give all my attention to the work at hand, get a sense of closure, and only then move on. You need to find your own comfort level; if you feel too pressured you'll lose that lovely creative flow and portraits will become just another job. Horrors!