Day to day exposure to sunlight will cause anything that is not 
a “noble” element to oxidize and break down. Modern resins have shown only a small gain in durability over the oleoresins that provide gloss to oil base enamels. Most of this gain has been in resistance to abrasion, afforded by linear urethanes and catalyzed epoxies. Signs don’t usually receive the kind of wear a yacht hull or car door gets. A car finish can last 15 years, but needs to be regularly washed, polished, and waxed, to last that long. Your sign finish needs a little maintenance too.
Wash with hose and soap. This is really all you need to do, once a year. Dirt, especially when mixed with oil, rain, asphalt and exhaust, creates an acidic paste that will collect in all of the horizontal surfaces, joints and seams and needs to be removed. Rinse this off, maybe washing with a sponge or soft cloth and mild detergent and the sign finish will probably rival a new car for longevity.
If your sign includes gold leaf, treat it gently. Gold will last decades outdoors so long as it is not rubbed off. Remember that it is pure gold, and soft. The grit from the road could scratch it, so make sure the surface has been well rinsed before hand washing.
