How much quilting does a quilt require? That depends on both the style of the quilt and also the batting used. For this 1930's era Sunbonnet Sue, the designs chosen should keep with the original designs that were quilted during that time. For example, grids, outlining 1/4" from stitching lines or applique shapes, clamshells, feathers, etc., are all keeping with original design work.
The back of the Sunbonnet quilt shows the stitching alongside the applique. A next step might be to quilt 1/4" away from the design to outline it. In addition, a line quilted 1/4" from the sashing, in the background area, would add additional quilitng.
Here shows the back of the sashing quilted in-the-ditch. An additional line could be stitched through this area to quilt down the sashing, if desired. However, it is not necessary because the batting used allows quilting lines to be 8-10" apart. I do like to cut that number in half and really have no space that is more than 3-4" open without quilting.
Back in the 1930's the batting used was 100% cotton. If the quilting wasn't heavy enough, the batting would break down quickly during wear and washing. Several of today's battings have what is called a "scrim binder" which is something like a grid which the fibers are bonded to. This makes a tighter weave inside and allows for looser quilting. Heavy quilting with this type of batting (i.e. Warm & Natural) can also make the quilt feel stiff, so check for how the quilt drapes over your lap while quilting.
So when is enough enough as far as quilting goes? Flip the quilt over, as shown here, and take a look at the amount of quilting already completed and see where there are spaces that still need quilting. Also, check the package of your batting choice and see what is recommended for spacing the quilting lines. Stitching through a sample of your fabric and batting is also helpful.