I just heard from a Tombow rep today about how well Tombow Dual Brush Markers resist fading. She told me that because the markers are water-based and not light-fast, they will fade over time if exposed to direct sunlight. The speed with which the pigments fade depends on different variables such as how much time they are exposed to sunlight, marker color etc. While she could not give me a specific fade rate for the colors, she did confirm that they stay very well preserved in areas with regular, indoor lighting and that Tombow has a framed painting done with Dual Brush Pens that looks perfect and that it has been there for over 10 years.
It sounds like they behave in a similar manner to watercolors so similar care would be appropriate. To be sure your marker illustration stays vibrant, care for it as you would care for a watercolor painting or perhaps a hand-pulled print. Consider framing it with UV protective glass and displaying in away from direct and indirect sunlight for the best resistance to fading.
To test which colors fade the most quickly, apply the colors to a piece of paper along with a label defining each color. Tape it with the color side facing out in a window which receives direct sunlight for a week or two. When you remove the test page from the window, compare it to fresh sample of the colors to see which colors have faded the most. You can keep this page with your markers to help you choose the best colors for your next project.