I had a king size quilt done on a long arm. Since I was in no hurry for the quilt, she had it for about a month. It really depends on how much time they spend at it and how soon they want to get it done and be paid. I would ask the lady how long it will take, she'll have a good idea about the time frame.
I've only been able to quilt baby quilts or small lap quilts on my machine. It's just too frustrating for me with anything bigger and becomes more of an endurance test for me than a relaxing pass time.
My dream is to purchase a new or used deep throated machine this year, either a Juki or Janome and a Grace or Handiquilter frame. Still checking things out. I think I would be able to actually enjoy the quilting part then.