My longarmer books 6-8 months out. She's a friend and is well worth going to, so I book as soon as I know for sure the quilt will be made. For instance, as soon as the Hoffman Challenge fabric was revealed last month, I booked my quilt. The fabric isn't available yet and the quilt design isn't done yet, but my entry will be quilted in April.
I never leave quilts with her for months and months. I take them to her when they are scheduled. Why should they sit in a bin at her house when they can sit in one at mine? That makes no sense to me at all. If she's ready for it a bit early, she calls me to see if I'm ready for her to start on it. If I cancel, she easily fills the time slot. She has over 200 customers and quilts full-time. The whole system meets the needs of both of us very well and I couldn't ask for a better longarmer.