Cathy -- I am quilting on every seam, so it is at 4" apart. My serpentine is a #4 stitch on my Bernina 430, shown in the picture. All the rows one way are done, need to go back the other way.
I really like it, it holds down the seams and you don't have to be so precise with stitching exactly in the ditch, which is great especially when moving such a huge quilt around. I use this stitch with all my charity quilting.
And to those who have asked about my size for a king bed, I just threw it on the bed and wish I had made it a bit wider. Another row of 5 of the the 16-1/2" blocks would have been perfect. Need to figure out if I want to take the one side apart and add another row. (The OCD will probably kick in and will add it.) Idid it 6 wide by 5 long, and the length is perfect hanging over the end of the bed, but seems a bit skimpy on the sides. Should have made it 7 x 5 blocks.