There are four enemies of photographic wall art.  These are light, heat, atmospheric gases, and the chemicals contained within the printing and mounting materials  themselves.  In recent years two processes have come to dominate photo printing - inkjet and laser or "lightjet."  In both cases the pigments and papters used to make these prints have evolved to a point where considerable longevity and resistence to color and tonal fading are possible - well beyond the lifetime of the original purchaser and even their heirs. However, it should be noted that even the best prints will not retain their color, vibrance, or their dynamic range if they are improperly mounted and displayed. 

Light is perhaps the worst enemy of photographs because of it's ability to bleach and fade an image.  Glass provides some protection from both ultraviolet and infrared radiation.  However, by displaying your photographic images out of direct sunlight, you can dramatically limit the effect of the sunlight's damaging rays.  The rays will be bounced and diffused off of other surfaces within a room before they reach the image, and the glass covering the image will dramatically limit any residual effects of this diffused sunlight. 

Atmospheric gases and heat are probably the two next biggest enemies of the colors and tones in photographs.  Atmospheric gases are absorbed by the fibers in photographic papers, and the chemicals in these gases, in turn, combine with the pigments in the print to inevitably change it's colors. I once saw a poster image that had been brought into a frame shop for reframing.  It had some kind of transparent plastic over it's surface that had been damaged with a small hole in the center. It had hung like that for years and now had a distinctly visible yellow discoloration on the surface of the print that radiated from the hole. The print had been hung out of direct sunlight in a cool, dark room, but had been changed by the atmosphere in the room. In addition to the effect of gases, heat breaks down the molecular bonds found in pigments and can also have a deleterious effect upon colors and tones.  This is why it is always best to hang important art away from heat registers, radiators and so on. 

As indicated above, today, the negative influence of the chemicals found in the pigments and papers has mostly been eliminated. However, prints that use poor materials for mounting and matting are still potentially subject to discoloration and fading through absorbtion of the chemicals from these materials over time.  For this reason only the finest materials should be used for mounting and matting framed prints.