Metallics, for whatever reason, do not last as long as their non-metallic counterparts.
Estimate life is a comparative rating against other films the manufacturer choose to test and compare to. If film #1 is rated at 5 years and film #2 is rated at 7 years, neither is a guarantee and it is simply a statement by the manufacturer that film #2 will last about 40% longer than film #1 in whatever environment the film is exposed to. So if film #1 fails in 24 months in a given environment, you can expect film #2 to fail in a little over 33 months. One should also take these ratings with a grain of salt because they are used as selling points in their advertising.
To attempt to make a fluorescent last longer is a waste of time if what you want is for the fluorescence to attract attention. It is the physical nature of fluorescent materials that they react with UV light and consume themselves in the process. If you reduce the amount of UV reaching the fluorescent material, you extend the life but at the expense of the reaction you are looking for.