The fire safe (as is typical for the class) should be rated for 1500 degrees for 30minutes while keeping the inside temperature below that necessary to char paper. The walls are heavily insulated and the seals on the door in extreme heat melt and seal the interior completely.
There are other things to consider which can seriously alter your fire situation.
(I'll point out that I have to do fire training including a number of evacuations through burning buildings every couple of years, and have been doing it since we routinely had people die in the training. It is very likely that I've spent more time in burning buildings in BA sets than the average Slashdotter, unless there are a lot of undeclared firefighters on the board. I work in the oil industry, and dieing in a fire is a non-trivial risk at work, in addition to the normal hazards of being at sea.)
Being in the UK, we build our houses with bricks, mortar and concrete, with minimal wood. Rather different to the US, I gather, and I'm not sure how that would alter the progress of fires there. If I were to get a fire safe, I'd probably mount it by either excavating into the floor of my garage (concrete) and covering it's access with a paving slab (40mm thick) ; or I'd mount it into the concrete and breeze block wall of the garage along the party wall with the neighbours. Since I don't keep significant flammables in the garage (that is in the shed, outside ; that can burn and I'd get a toasting fork instead of getting worried.) Either of those options should take hundreds of degrees off the exterior temperature of the fire safe. Putting it in the floor would be better temperature-wise, but not so good for protection from water damage. But since I've also got water-proof diving pouches and silica gel in abundance, I'm pretty sanguine about water.
Where to site a fire safe - for a combination of low fire risk and acceptable security (I wouldn't put it in the shed!) is going to have a very big effect on the conditions it is exposed to.