Our Mavics (might have been 521s, don't remember) lasted a really long time. The first one to do was mine in Syria, when we'd done about 15,000km (maybe more! don't know without looking it up). And the only reason that one failed was because my brakes weren't adjusted right for a while and the rim wore down unevenly. We replaced the Mavic with a cheap-as-chips single-walled rim from a shop in Syria (very basic) and that went all the way through to Yazd in Iran (so another 5,000km or so) before we replaced it.

At that point, we replaced all the rims, and they all had small cracks starting to show on the inside. So, I guess if you've done a lot of KMs and are curious, just take your tyres and tape off and have a look to see what's happening in there...