How many of you do commission quilts for others? How long have you been providing this service and how much do you charge (do you charge based on size ~ flat charge etc)? What kind of turn around time do you offer?

This is one area that I specialize in. Custom one of a kind quilts.
#1 Depends on the complexity. A queen size double wedding ring, piecing only, starts at $800 but a queen log cabin starts at $400.
I start my calculations at 25cents per square inch, add 25% for extras, quilting etc.
Tshirt quilts go for $25 per shirt.
One thing you need to take into consideration, you charge according to what your community will pay.
I tell my customers to give me at least one month. That can go as high as 2 months depending on what my longarm business is doing at the time.

I used to - mainly wall pieces for offices, and charged for the artistic value. I would charge $25 for the sketches, then materials +100% to assemble them. Sometimes I made money, sometimes I didn't. It's good to have everything in writing and make realistic deadlines. If I was doing a story quilt, it would take some serious time to make patterns alone.

The area you work in is really important, as already brought up. Where I am now, you can barely give quilts away. There is zero concept of fiber arts, just thought of as a pastime for old ladies. I've had gallery owners tell me they do not touch quilts, even as consignments, and in extremely disparaging terms, too.

I am making a tee-shirt quilt for DH with his hiking and work tees. I just told him it was $25 per shirt. LOL I was going to use the shirts front and back cuz he has so many. I am going to be rich!!! Thanks Sharon.

I make the tops and "Vickie" does the machine quilting. (I don't know if you're asking about hand quilting or machine quilting.) I furnish the top and backing-make sure the backing is at least 6 inches longer than the top. Pricing for free-hand, custom, and special designs is 10 cents per square inch. There is a 3-week minimum time time. Pricing includes batting and thread. Hand-sewn binding is available for 10 cents per inch. Here is her current price list for your consideration:
Baby: $20.00 36" X 45"
Lap: $25.00 45 X 72
Twin: $40.00 70 X 96
Full: $45.00 80 X 96
Queen: $50.00 90 X 100
King: $55.00 100 X 100
Super King: $ 60.00 110 X 110

Vickie lives in southern Missouri. I mail her my quilt tops from Kansas City :)