When a safe goes through a fully developed fire the heat activated door seals will swell up and seal the door quickly (assuming they are good quality). Over time, the peak internal temperature will gradually climb up to about 220-230 degrees F and then plateau for awhile. The peak temperature will remain at that 220-230 degrees F range until all the moisture is cooked out of the sheetrock fire lining. I say "peak" because there is usually about a 80-125 degree F difference between temperatures at the top of the gun safe and at the bottom. (Depending on height of safe and design)
After the moisture is cooked out of the sheetrock, the internal temperature will again begin climbing at a pretty steady rate. So the key is having enough sheetrock to keep the internal temperature on that plateau. Beyond a certain point, multiple fires seals are also needed in case the outer most seal fails at some point during the fire.
Low quality fire seals, or any exposed areas inside a gun safe not covered amply with sheetrock, will create hot spots and decrease the duration of that plateau. A typical place to find exposed areas is on some of our competitor's safes with internal hinges, where the hinges attach to the inside door frame.
So armed with that information, here are some simple considerations that might help you now decide what rating is right for you. First, think about what your safe will potentially be exposed to in terms of temperature and duration. One of the biggest factors affecting this is the fire fighting response and capability in your area. Most urban areas have very fast response times, in which case, a 30 or 40 minute fire rated safe might be adequate. Next consider the fire fuels in and around your safe. If the safe is in a garage next to a bunch of gasoline cans, oils, and other flammable items, then you probably want something at one hour or more. If you think your house might burn to the ground, then the 120 or 130 minutes ratings would be appropriate.
Another consideration should be how heat sensitive the contents of your safe are. If you're storing some photographs or electronic data then you'll need to make sure those items are kept cooler than the rest of the safe. Typically, electronic data, like CD's need to be kept below 125 F, so it's best if you keep them low, or even better yet, put them in one of those lockable fire resistant cases (sold at Walmart for about $16) on the floor.