A properly installed underground propane tank should last at least 40 years or maybe even 50 years so the underground propane tank can last a long time after being buried.
My propane tank is buried underground and it's supposed to last at least 40 years according to my propane company so I will 80 years old when it likely needs replaced.
I like the look of my propane tank being buried underground now instead of being above ground and it gives me more space in my yard.