Pretty darn durable. We use aluminium for our under tray boxes in Australia as it has comparable strength and durability to steel while only being a fraction of the weight. We also install a weather-resistant seal and heavy-duty hardware to ensure that a bit of rain or some fool looking for an easy break-in won't be able to get at anything either. That said, make sure that you're maintaining it properly. Keep an eye on the seal, make sure you're keeping the interior clean and oil the lock regularly – it should last you for years if you're giving it the TLC it needs.