Fumoto valves can easily outlast the life of your vehicle. Many customers even remove their valves from a vehicle they're selling and install it in a new vehicle if the sizes are the same.
With first generation valves, however, the gasket washers may need a change every few years. Sometimes they'll be fine for well over a decade, sometimes they'll take a beating. It truly varies by car and driver.
You can always purchase a few extra, just in case.