We have clients with storefronts that have been on for over 6 years and still look great. There are a lot of factors that can influence the longevity like direct sunlight, awnings, cleaning, etc.
Your view through storefront graphics should last at least 2-3 years. They will still look great after 3 years but can begin to crack or fade. Solid (non view through) graphics can last longer. Die-cut vinyl lettering can last 8+ years.