I just finished hand quilting a quilt for my daughter. I started quilting it about 4 years ago. I am not a fast quilter. I used the blue marking pen and the marks were there for about 3-4 years.
When I finished quilting I held my breath and started spraying with plain water.....and the marks all came out. What a relief!!!
Every time I stopped quilting I folded the quilt up with the top on the inside so there was no light getting to the markings. I don't know if this helped, but the water soluble pens do work.