I feel comfortable allowing 3 to 4 inches on each edge on my midarm for two reasons. Since I use a variety of threads, it is important for me to check the tension before quilting. A scrap is laid on top of the batting/backing edge so that I can adjust the tension before getting on the actual quilt top.
Once the quilted top is off the frame and squared off, the leftovers are usually wide enough to make strips for binding and/or jelly rolls.