We've had emails and comments lately asking us to categorize, objectify, predict, algorithm-ize, and other wise say how long rims last. If there is a better "how long is a rope?" question I don't know it, but I'll take a stab.
The reason I say that this is a "how long is a rope" question is because no two experiences will be the same. I typically have one really hard braking incident per ride, and that's if I get stuck at a light. Apart from that, my brakes are more for speed adjustment. During the time of the year when you actually want to ride, it doesn't rain too much, but there's plenty of salt and sand to go around when it is wet. At 167 lbs, I'm neither light nor heavy. When I use rim brakes, I use KoolStop Dura 2 dual compound pads, and when I use carbon rims I use the recommended/mandated pads. SwissStop BXPs are also exceptional pads for alloy rim brake rims, and typically mandated for ceramic (not ceramic) rims.
That lays out most of the variables involved in how much stress you're going to induce into your rim's braking surface - how much you brake, how much you weigh, what pads you use, and how much you ride in wet. And the wet isn't really the wet part, it's the gunk the comes along with wet. There's also the question of how durable are your brake tracks? One of the big benefits of some of the rims we sell is that the alloy is harder and more wear resistant. The ceramic (not ceramic) coated rims are really hard (but they chew up some pads getting going). HED Belgium+ and Easton R90SL are also really resistant to wear. Kinlin rims are good, but the alloy they use is a bit softer and more prone to wear. DT Swiss has a range of rims with different alloys - the sleeve-joined rims are generally lower spec alloys that will wear faster, and the welded rims are typically harder, more resistant metals. Call me a crackpot, but when I lace a wheel I can tell by the general demeanor of the spoke holes how well the thing will brake and resist brake wear.
Knowing when a rim needs replacement is generally pretty easy in alloy rims. Hold a straight edge up to the brake track and see how much cupping you get. The rim in the photo above should have quite a bit of life left in it - depending on the rim I'd say that's about half way. Lots of rims have a small dot in the brake track (which a lot of people think is a defect - it isn't) and that's a wear indicator. When you no longer see that dot, replace your rims.
Carbon rims are a little tougher to judge. Carbon rims don't cup as readily as alloy rims, but the cupping they do is more harmful. So if the pic above was a carbon rim, it would be much closer to end of life than the rim than it is as an alloy rim. But because carbon is way harder and abrasion resistant than any aluminum alloy that I know of, it would take way way longer to get to that amount of wear. A better test for carbon is when you see the brake track start to look "disorganized." The top layer of carbon in a brake track is typically (but far from always) a woven material these days.
Eventually, you'll wear through that woven layer. Because of the nature of woven things that have the over/under... you know... woven thing going on, you're going to wear sort of a half-layer first, and it will still look generally like it did at the start. After that, there will typically be a structural unidirectional layer (like you see in the pic above, in the part that's not the brake track), and that will start to show through. Even if the layer underneath is still woven, the orientation of the weave will be different, and it will be noticeable when you're through the top woven layer. When that happens, start rim shopping.
Now, how long do hubs last in relation to rims? Just taking a set of White Industries T11s as a straw man on this - forever. I mean, eventually the end cap fit might get a bit sloppy and you'll eventually replace some bearings, but those are easily replaceable components. This is a huge part of why we're doing the Rail 55 - the Rail 52s and 34s that we've sold are generally getting long in the life span here, and the hubs are still good. Notice the BYO hub option for ordering. OEM hubs will start to get distorted in the hub shell over time, and they might be worth rebuilding or they might not. Certainly in the case of a premature rim end thanks to a crash, an OEM hub is going to be worth rebuilding into a new rim.
How long do disc brake rims last? Barring damage, eventually the spoke holes are going to go, but I would say start with 2x the equivalent life span of a rim brake rim and go from there.