depends on A- the actual size of the Qn sized quilt, B- the type of stitch used to stitch it down, C- the age of the person doing the stitching, D- the expertise of said person, E- the # of people in the household who demand dinner, dishes, laundry, etc be done, and so the list goes on....
I use the ladder stitch so it takes a bit longer, my needle hand also has arthritis, so that slows me down, I have dogs that can not let me sit more than 1/2 hour before one of them needs something..etc..etc...it takes me about 6 hours to do an average QN quilt!