I recently read an article written by an interior designer, in which they had mentioned that oil paints last longer than watercolor, which may be the case when looking at antiques, but does not hold true today. I have done a bit of research that actually say watercolors last as long as oils. What it really boils down to is the quality of the painting as well as the condition in which the painting is kept, no matter what medium.
The quality and technology in today's paints (all paints) and support materials are much better than even years ago. I use quality paints*, paint on an acid free watercolor support, and use archival quality framing (100% ragmat and acid free foam core), with UV protective glass. As with any painting in any medium—or photo or print, it is wise not to hang the piece in direct sunlight, or in poor conditions.
*I paint with A or AA rated pigments, which are rated as "permanent" (from the Winsor & Newton website: AA - Extremely Permanent, A - Permanent, B - Moderately Durable, C - Fugitive).