The light fastness of a litho ink depends on the pigments used; only the ink manufacturer can provide information about a particular ink. Ultraviolet (UV) light causes the pigments to fade; the amount of UV light exposure will have a direct bearing on the amount of fading. Generally, synthetic pigments fade sooner than natural pigments in UV light. Ink manufacturers are reluctant to give specific time guarantees for light fastness as the amount of UV exposure can vary greatly from the location of the printed sample. I have heard of ink manufacturers using automotive pigments for the most demanding fade-resistant inks, but the larger pigment particle size limits the choices available for litho inks. Most manufacturers will not guarantee light fastness of litho inks over one year. We have a fadeometer in our lab that simulates exposure to UV light and can test ink samples for fading.