Rims wear out from brakes rubbing on the rim. Failures may surprise riders who ride mostly in the dry, and sometimes surprise even riders in wet areas.
Here is a worn rim. When new, the sides of the rim were approximately parallel.
Now, the brake track is quite thin. The diagram suggests it was originally thickest between hook and web, thinner between web and spoke bed. Now, it is thinnest near the web, and also thinner than the sloping parts of the spoke bed, although the diagram suggests the brake track was originally thicker than the sloping parts.
This rim was replaced before it failed. If it had been used more, one of two failures would be likely: either it would crack at the thinnest part of the box, and the wheel would suddenly loose true and develop a radial "lump"; or, the part with the hook would crack off — sometimes, such cracks lead to a lump, sometimes the tire pulls hard enough that the hook tears off a long section of the sidewall, often as much as ¼ of a turn, and the tire goes flat with a bang.
Rim failures can strand you, and sometimes hurt you. So it is a good idea to replace worn rims. However, wear is unpredictable, making it hard to just replace rims according to the calendar. For example, one rider often gets 50,000 km from a rim, and replaces it due to flat spots and other damage from hitting obstacles, not from brake track wear. Another rider gets as little as 500 km during wetter parts of the year.
Some things that affect wear-out rates include:
For example, the rider above who gets 50,000 km is riding mostly on-road and during rainy weather tends to ride a fixed gear, and thus avoid brake use. The rider getting 500 km is riding off-road in wet/sloppy conditions that coat the rims with mud, is fit and fast from lots of on-road cycling, but is a novice off-road cyclist so often brakes more than a rider with extensive off-road experience.
A further detail is rim failures depend on the tire and rim sizes and tire inflation pressure.
Since "replace by the calendar" is impractical, it is important to look for signs of wear before you are stranded or injured. A typical sign of wear is the inward curve of the brake tracks, shown in the picture above. (There is some parallax distortion from photography, but the gray bars on either side are parallel.) The "hollow" happens for two reasons:
Many newer rims have wear indicators. One common type of wear indicator is a groove in the sidewall; when the groove disappears, the rim is worn out:
Many rims lack wear indicators, but can be checked easily for wear. Many rims start new with sides that are nearly parallel, and you can check for wear by putting your thumb on one side, a finger on the other, then moving your hand radially between hook and spoke bed. If the width changes substantially, it is likely time to have the rim checked by a mechanic. Checking rims without a wear indicator would be better if makers published nominal minimum widths across the brake tracks, and a maximum width at the hook, to check for spreading.
Not all rims start with parallel sides. Some rim models start new with the brake track curved as shown above, wider at the hook than the spoke bed. A few start new with the brake track wider at the spoke bed than they are at the hook.
Here are some rims with worn brake tracks: