A pretty broad range actually. They tolerate temperatures as low as 35F in the wintertime. They rank among the most temperature tolerant of fish.
At the high end, goldfish are pretty much like people. Temperatures in the 70s are ideal, but they can tolerate the 80s with no problem, and distress does not set in until the human normal temperature of 98 is exceeded. Incipient lethal temperature is 37-40 degrees Celsius (98.6 - 104F), which means that prolonged exposure to 99 or brief exposure to 104 can prove fatal. Just like us.
I have one pond that receives direct sunlight on a south facing wall, and during heat waves the water temp often hits the mid-90s during the afternoon. The fish do fine, but they swim less and tend to hide in the shade. The mercury dips back into the 80s at night.
High temperatures cause two other problems: low oxygen, and heightened toxicity for any ammonia present. The warmer the water, the less O2 it can hold. This especially causes a problem at night, when plants do not photosynthesize CO2 and therefore use oxygen. If your water is going to go above 85F, use an aerator in addition to a waterfall/filter flow. Also, as temperature and pH rise, so does the harm that ammonia can cause. Be sure your filtration works and do frequent water changes. Also replenish evaporated water, which can be substantial during the summer months. Remember, replenishment does not substitute for water changes!
You should be fine if 1) your filter works, 2) you have adequate aeration, 3) you have some plants with floating leaves for shade, and 4) you keep your water changed.
Just in case, freeze a couple of liter bottles of water to put in the pond if the water temperature exceeds 95 degrees.
