I did a road ride from Winthrop up into the local hills there; it was 4,000 feet elevation gain in 20 miles, and it was out in the desert. Temps hit about 100 F. I have carbon rims, including the brake tracks. On the descent, I lost quite a bit of braking power. My wheels "scream" when this happens, I hit the brakes and don't slow down much, but hear a loud noise.

Like you, I pull the brakes hard, but for a short period of time. Get rid of some speed. Then, I let go, picking it back up, and the wind cools the rims a bit. I really don't know the ratio, but it might be 10 seconds of braking and 1 minute before the next time? Really it depends on the terrain. I don't give it everything I have, in case I need to make a surprise stop soon. Most mountain descents haven't given me heat trouble, but I actually stopped a few times to let things cool on the way back to Winthrop. I also went through a lot of curves without slowing down first, letting the brakes cool for my next steep "pitch."

I get uncomfortable around 35 mph, typically. Most of my riding happens around town (weekend escapes lately have been for backpacking and climbing), and you never know when someone will pull out without looking, or a dog or kid will run into the street. Knowing I have good brakes, good tires, and a stable bike helps with the mental part.