First of all, the most important part is that the inside of the box remains not compromised. On the outside a safe can get burn marks and other visual traces. But that doesn’t necessary mean that the internal part is damaged. Safes are designed to preserve the valuable content not to maintain perfect look. Even though some higher-priced models do aim for that as well. So when a customer wonders for its longevity it usefully pertains to preserving the contents.

Now back to the specifics. If you take a UL Class 125 2-hours certified safe as an example it will last for at least 2 hours as long the external temperature does not go above 1700°F (927°C). And the average home fire usually reaches around 1000 °F. This means the fireproof safe can last even longer. With this type the inside temperature will not rise above 125°F (51°C), meaning you can store electronic devises inside and other computer media.
UL Class 125 1-hour examplar will last at least 1 hour under the same conditions. But if the house fire produces higher than accepted heat ranges, the inside of the storage box will reach higher temperature thus damaging the contents. It is impossible to say how long it will last in this case.