I mounted new rotors a week ago, and today they make a crunching noise just as terrible as on day one. It's a sound that makes you think a small rock is stuck between your pads, except there isn't. I did maybe 100km on the rotors, exclusively city rides with no significant descents, but I still feel like the crunching (which I've read is normal at the start) should have stopped by now.
What kind of break-in time is normal for new rotors? Should I be worried something is wrong?
Rotors take only a short time to bed in, as Paul described in comments. However, you need to actively bed them in. This means you need to execute a few hard stops. You could drive up a hill with the bike, then coast down, and pull the brake levers hard a few times. Or sprint and then hard brake several times. I’m not sure if the rotors actually bed in if you just ride normally. Even if they did bed in, you’d be suffering from poor braking until they did so.
If I recall correctly, the symptom of unbedded rotors is lack of braking power. It’s not noise. Without being there, it’s impossible to be certain, but the noise you describe sounds more like there is actually some debris stuck to the pads. You could take the wheel out and perhaps wash that out with water or blow it out with compressed air.
Normally, I believe we usually wear the pads out before the rotors. So, if this is a relatively new bike, I am slightly surprised that you changed rotors but not pads. Ignore this if you actually measured your rotors and pads and the rotors are below the minimum thickness. However, if you had worn the pad material off completely, then when you brake, the bare metal pad holders are going to be scraping the rotors, which is bad for the rotors. I think this isn’t likely to be the case, but I wanted to warn you just on he off chance.
I now can say that in my case, it took 8 months and approximately 10000km, but finally my brakes as silent as disc brakes are supposed to be. I'd say about 40% of the noise went away during the first ~10 days, but the other 60% was extremely persistent and only really dwindled down during the last few weeks.
Brake pad changes didn't affect the noise at all, so I'm sure that the rotors actually were the cause. My best guess is that the factory saves some of the money on properly deburring the edges of all the holes in the brake surface, so the pads had to slowly grind their way down.
By the way, I live in a decently hilly city with a few steep descents, so the brakes got more than enough use. IIRC I used up 2.5 sets of pads since I mounted the rotors.
What kind of break-in time is normal for new rotors?
One data point from me: new SM-RT64 rotor and new K05S-RX organic resin pads, at the same time. The bike wore away two sets K03S organic resin pads and one set of K05S-RX pads, and wore away the first SM-RT64 rotor to 1.5mm thickness, in 10500 km, after which I changed both pads and rotors.
I did notice that the combination of new rotors with new pads was noisier and less powerful than the old rotors and old pads (same models, just used parts) were.
Initially, the braking performance was unacceptable to me. The noise was awful.
After about 15 km of riding (from a suburban area to an urban area but not city center), where I braked hard every time I had to brake, the braking performance increased to the point I consider it acceptable. I'm not still sure if it's equal to what it used to be after 25 km total of riding, maybe slightly lower (at least it sounds slightly more noisy still), but this is a level of braking that I wouldn't be cautious about: I can finally stop without worrying if I stop quickly enough.
I think the break-in time is likely (negatively) exponential: most gains are in the first 1-5 km, then the rate of gains becomes less, and it may take a really long time until you have the same level of braking and same level of noise you used to have.
I did maybe 100km on the rotors, exclusively city rides with no significant descents, but I still feel like the crunching (which I've read is normal at the start) should have stopped by now.
So you still have more noisy braking. Do you have unacceptable braking power? For me, the unacceptable braking power went away in 15 km, but in 25 km there is still somewhat more noise than on the old parts.
Brake noise is something that you may have to live with. It's just a fact of life that brakes sometimes may emit noise, maybe a little bit of squeal (but continuous loud squeal isn't good), maybe the pads sound like sandpaper. But if it stops you in acceptable braking distance, it's good.
My old parts sounded like a high frequency vibrating noise, my new parts sound like sandpaper with no more high frequency vibrating noise. The new parts are probably somewhat noisier on a decibel scale, but the type of noise they emit is markedly different. The high frequency vibrating noise may have been caused by wearing the first set of pads so close to metal that the metal pad spreading spring may have touched the rotor (causing me to realize the pads needed changing).
"they make a crunching noise just as terrible as on day one. It's a sound that makes you think a small rock is stuck between your pads"
This makes me think you have either contaminated pads, or you didn't "reset" pistons before replacing the new pads. Probably the latter...
Remove your pads, push pistons fully in, and reinstall again. If problem persists, try replacing them with the pads at back. If sound gone, it's contaminated pads. If there is still noise, maybe it's time to bleed your front brakes.
Most of the time, those noises are a manifestation of not enough brake force.
