Interesting question. In a lqs, I purchased a fabric that was a good 20' from the windows. As the fabric was unrolled to cut, I noticed a faded part at the same spot - so about every 10 - 12 inches or so - with each unfolding. It took about 4 'unrolls' to reach the part of the fabric that was not exposed to daylight while sitting on the shelf. The fabric did not get direct sunlight.
It is possible that at one time it was closer to the front of the store, though. Even so, the fading had penetrated farther than I ever imagined it would.