I was wondering how long it generally takes for disc brakes to overheat, if you descend very slowly and therefore use the breaks a lot.
Impossible to say without a lot of specific info. Factors like rider & bike weight, road pitch, ground speed, disc rotor type, disc pad type, presence or absence of cooling fins on pads a/o rotors and such will all affect the info.
Generally speaking, unless you are doing prolonged descending with brakes on hard and consistent, you won’t heat them excessively. Is there a specific question or concern you have besides that?
A useful technique to use in some cases of long braking, is to “cycle” the brakes with a relatively hard application to slow down, release the brakes for a moment to allow the pads and rotor to cool, and then reapply the brakes. Rinse & repeat down the hill. This can help cool the components a bit vs sustained loading.
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Also alternate between front and rear if you want to maintain the same speed on the way down and not vary your speed
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I was surprised recently whilst doing the Fred Whitton in the Lake District in the UK at the number of people running into disc problems due to overheating. The Lakes are known for some brutally steep climbs (e.g. Honnister, Hardknott, Wrynose) and there’s some sketchy steep descents which require a good amount of braking for most. My friend’s disc warped due to the heat and a few others noted exactly the same issue. Not the kind of descents where you’d be on the brakes continuously but some very hard breaking required at points, so it was fairly unexpected. I was alright on rim brakes, thankfully it was dry.
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Thanks for your inputs guys!
I’m going to do the Haute Route Alpe d’Huez (Compact), and noticed that the first day has 2 long descends in it. It starts with a 22 km descend down Alpe d’Huez, than the ascent up Col de la Croix de Frer, and ends with a 22 km descend down the Col de la Croix de Frer before finishing up the Alpe d’Huez. Some areas appear quite steep, Komoot gives up to 25% inclination which freaks me out. I expect to go down-hill extremely slowly, implying a lot of breaking. Hence my question how long it generally takes before disc brakes overheat.
My ride is a Domane SL7, the brake rotor is SRAM CenterLine X, centerlock, round edge, 160mm, the bike weighs 8.9 kg and I’m about 75 kg.
I’m planning on changing the brake pads in two weeks time, and bringing along extra brake pads just in case. But that won’t help if they overheat during the ride on the first day.
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There seem to be a bunch of pads to compare: I can’t find the Shimano L03A brake pads anywhere what other options do I have? - YouTube
Some interesting temp numbers here:
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Our tandem has an XTR v-brake as a drag brake for exactly this reason. We run 203mm discs front and rear with floating rotors and finned pads and still overheat on long descents. Bit terrifying the first time it happens!
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the descents in the alps are generally pretty pleasant. I would guess the 25% inclination is on a switchback? Most descents in the alps if you can coast along the straights and check your speed with the breaks before corners, so the pads should have plenty of opportunity to dissipate heat.
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I’m 90kg and have heavily used the brakes on some long mountain descents with switchbacks. One of my earliest was also the longest at 21km - the brakes were howling about 10 minutes into the descent. My idea of overheating is when the brakes start making howling noises - resin or metal pads.
What I learned during that descent:
That’s worked well for me on other descents down steep & switchback HC climbs. I never had any reason to think of changing the brake pads during the ride. All of my bikes have had 160mm rotors.
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I desdended stelvio on a city hybrid bike with relatively cr*ppy MTB hydraulic brakes and lived to tell the tale. I think rotor size makes a massive difference to cooling, I used pretty basic AliExpress 160 mm rotors.
One thing to note, one of our party was an extremely slow descender so I waited once or twice for him. There was lots of water in the gutter because of snow thaw, I filled a bottle with that and sprayed the rotors and it would come up as steam! Maybe this helped too 
For reference I used to weigh 86 kilos back then and I was doing some lightweight touring with 4-5 kg of luggage, and a 9 kg bike.
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popping tires is better as the air pressure goes up from heat?
Bike Virginia one year there was someone with rim brakes on carbon wheels where the wheel disintegrated on the decent. I was far behind everyone else so only saw the parts of the wheel on the road afterwards.
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Race cars use pad compounds designed to handle high heat and they guide tons of cool air onto the rotors. But, its a trade off since usually that almost always means the brakes suck when they are cold and you can actually get in trouble if you don’t keep them hot. I’ve never looked but I’d assume there are bike brake pads that handle heat better than others.
FWIW, brakes in a high end race car routinely glow red and still perform very well. They just might be undrivable in stop and go driving around town though . . . .
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There are a number of guides around for considering different pads:
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We’ve got aluminium rims on the tandem (it’s a 26" full suspension MTB tandem), so the drag brake doesn’t heat it enough to cause any issues with heating the tyres enough to pop them. I’d be more concerned if it was a road tandem, especially with carbon rims!
I would probably avoid doing this as it can cause the rotors to warp
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I agree, I just didn’t know any better
I just wanted to illustrate how hot the things went
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This guy on YouTube has an adapter so you can mount 180mm rotor on your road bike (he goes over the size your fork needs to be to accept his adapter).
I wonder if this would be something that would work and give peace of mind?
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At $400 a pop it’s definitely a boutique item.
That was HKD400 actually, which is about USD50. Much more reasonable.
So not titanium? 
How am I supposed to put up with the extra gram of weight?
I wonder if using thermal paste from computer use to mount the rotor onto the hub will help transfer heat to more of the wheel
I know it’s just a joke, but titanium is actually twice as dense as aluminum.
No, because the the stainless steel used to make the rotors doesn’t conduct heat as well and the small cross sectional area of the “spokes” of the disk bottleneck the thermal transfer anyway.
Which is actually desirable. You don’t want your hub bodies expanding from the temperature rise, causing your press-fit hub bearings to rattle loose.
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