Even if it does require billions of flashes to begin to fade artwork in a gallery setting, consider some of the most popular pieces of artwork, such as the Mona Lisa, which are heavily photographed and will continue to be so over hundreds of years. Over time, it's feasible that enough people would photograph it with a flash that it would begin to fade.

If the data and its analysis in this article are correct, it doesn't make much sense to ban flash photography in most settings, but it does for things which are and will continue to be extremely popular for centuries.