I am a New York based operation and are commissioned from clients worldwide.
Due to my portraits being in heavy demand, I have quite a long waiting list, often booked 8 to 10 months in advance depending on the amount / complexity of client's wishes. My "sketches" are usually about 6 to 8 weeks. The client list can always be viewed on my progression session blog on the left hand side.
I have a first come first served policy. The sooner I can lock you into my calendar with the deposit, the faster you get to the final product. However, if payment is made upfront in full, you cut the line from #45 to #10 and get you it around 4 month turnaround rather than the 10 months wait.