Well given the hugs number of really worn out BBs I've seen and that I see them often when the bike is brought into the shop for some other reason I would suggest that replacement time is totally subjective. We get so bound up in "standards" and "guidelines" when what really maters is the rider's perceptions and tolerances. Until the worn out parts cause other issues (like the crank/rings rock back and forth to a degree that this movement causes the chain to derail) worn parts can continue to wear yet "work".


I find the best way to gage bearing wear in general is to remove any levers involved then spin the bearing by hand and feel it's smoothness, or roughness. For a Bb this means removing the crank arms and turning the Bb spindle with your fingers. It's surprising how different a bearing will seem when spun by the end of the crank arm (or rim) compared to just the axle being rotated.


But in the end without X Ray eyes actual bearing track/balls conditions is unknown without taking all apart. I suspect the shop merely spun the crank, maybe even without removing the chain from the rings, say the rotation was not smooth and assumed that the ball tracks were pitted. This is a common quickie to do an initial assessment. Most all shops do this. And most shops have found that actual taking apart gives the real, no assumptions, answers.


BB, or any other bearing, wear is not solely a mileage thing. The initial set up, follow up maintenance, the load when riding, the conditions (rain, salted streets, mud) are all more important as the grade of the bearings, IMO. This is why one rider gets 10,000 miles+ before the need to replace and another rider gets less then 1000 miles. Andy.