This is a question for the ages. There is no single right answer that works for all quilts and all quilters. Do you need 3-4" all around for a longarmer? If not, how much extra do you need? What width of usable fabric can you count on? (I generally figure I can get 40", no more, but sometimes I'm pleasantly surprised and wind up with as much as 42".) Do you mind doing lots of piecing, or do you really want just 2 or 3 widths of fabric running down the back? (Keep in mind that a longarmer wants to mount the quilt with backing seams running across the poles, not up and down. If yours run up and down, can your quilt be mounted sideways and still be quilted with the design you want?) Is your fabric directional, or do you need and want to match motifs when piecing it? Do you have scraps from the front, or other fabrics in your stash that can be used to augment the backing fabric? Do you even like pieced backs?
I'm always amazed how long it can take me to make a quilt back. I shouldn't be, because there can be a lot of sewing, a lot of measuring, and since it's done without a pattern there's also a lot of thought involved.