Gemm, a few years ago I took a number of clear and pastel plastic under-bed storage boxes of fabric along with me to the lake for the summer. I kept them under the bed in a small bedroom with no direct sunlight due to the trees. When we moved back to the city in the fall, I noticed that the pieces on the side of the boxes that had been facing the window were faded along the folds, so it can happen quickly and without direct sun light. After that, I have always stored my fabric on shelves in closets with with tight-fitting doors. I had a friend that worked in a large fabric store and once a month they would rotate all their solid and dark fabrics end to end on their shelves because otherwise they discovered that they could fade from the neon lights.