The electronics are simple so much of it will depend on how well it's taken care of physically and the environment it's used in. The heaters all use resistive wire, typically a nichrome alloy. These work like an ultra-low wattage incandescent light bulb - converting electricity to heat over a distance. Except instead of converting 75 watts over a fraction of an inch, the different wire choice converts ~50w over 1+ square feet.

That's all relevant because of how things can go wrong. If an electrical short is caused by moisture ingress, the heater's resistance drops and it will suddenly pull a lot more power and often melt. Extreme hot and cold cycling can stress the plastic case as can UV exposure if they're left exposed to the sun.

About the worst possible thing to do is leave one of the pads on the ground in an area with poor drainage. It will eventually fail. But if it's kept within an enclosure and off the ground any of them should lead a long life.