I had to replace a Campy Record BB because I over-tightened it and cracked the cup. 60K miles and still smooth as butter. I replaced it with a Chorus BB and that one is still in use with well over 100K miles. Quality components can make a difference. YMMV
Sometimes it is difficult to tell; what you're trying to do is to unload the bearing and turn it with your hands and see if there is any rough feeling or odd sounds. Sealed bearings do wear out but when the proper sort are properly installed the loads on them are a great deal less than bearings are designed to withstand. However, cheap bearing such as "Enduro" wear out pretty easily because the balls and races aren't cut and polished correctly. If you have to replace them the best bet is to go to an auto supply store or bearing supply store and get the best bearings available. Given this and proper installation a good sealed bearing will last almost forever. Bearings pressed into carbon fiber bottom brackets such as Trek's awful BB90 are often misaligned from side to side which causes rapid wear friction in the bearings and then that beats the press fit all to hell. Any bearings like this which are in some form or another in most of the top end carbon fiber bikes should be closely watched for ANY sounds and if so take it to a reputable dealer. Bearings will not self align and instead will put undue pressures on the bearings that they are not designed to take. Trek has finally admitted their screw-up and replaced the BB90 with a T47 BB which is a metal screw in bearing mount.
